Partial pruning method for inter prediction

ABSTRACT

Techniques for video encoding and decoding are described. An example method of video processing is disclosed. The method includes classifying motion candidates in a candidate list into a plurality of categories of motion candidates, each category being assigned a corresponding pruning rule; performing pruning or skipping the pruning of the motion candidates in the candidate list based on the corresponding pruning rule, the pruning being applied to determine whether to insert motion candidates from the candidate list into a final candidate list for a current block; and performing a video processing on the current block based on the final candidate list.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Application No. PCT/CN2019/122828, filed on Dec. 3, 2019, which claims the priority to and benefits of International Patent Application No. PCT/CN2018/118896, filed on Dec. 3, 2018. All the aforementioned patent applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This patent document relates to image and video coding and decoding.

BACKGROUND

Digital video accounts for the largest bandwidth use on the internet and other digital communication networks. As the number of connected user devices capable of receiving and displaying video increases, it is expected that the bandwidth demand for digital video usage will continue to grow.

SUMMARY

The disclosed techniques may be used by video decoder or encoder embodiments during video decoding or encoding using candidate motion lists.

In one example aspect, a method of video processing is disclosed. The method comprises classifying motion candidates in a candidate list into a plurality of categories of motion candidates, wherein each category is assigned a corresponding pruning rule; performing pruning or skipping the pruning of the motion candidates in the candidate list based on the corresponding pruning rule, wherein the pruning is applied to determine whether to insert motion candidates from the candidate list into a final candidate list for a current block; and performing a video processing on the current block based on the final candidate list.

In another example aspect, a method of video processing is disclosed. The method comprises enabling a pruning process for a geometry partition mode based motion candidate, which is to be inserted into a final candidate list for a current block, based on a pruning rule, wherein in the geometry partition mode, the current block is split into at least two partitions which are non-square partitions or non-rectangular partitions; and performing a video processing on the current block based on the final candidate list; wherein the pruning rule depends on regular merge motion candidates from which existing geometry partition mode based motion candidates in the final candidate list are derived.

In one example aspect, a method of video processing is disclosed. The method includes generating, during a conversion between a video comprising a current video block and a bitstream representation of the current video block, a list of motion candidates, categorizing the list of motion candidates into a number of categories of motion candidates, wherein each category is assigned a corresponding rule of pruning and performing the conversion by performing the pruning using a pruning method according to the rule of pruning to decide whether a motion candidate could be added to a final list of motion candidates and decoding the block based on the final list.

In another example aspect, another method of video processing is disclosed. The method includes generating a regular merge candidate list by applying a pruning process to history based motion vector predictor (HMVP) candidates using a rule that is based on where merge candidates are derived from and performing a conversion between a current video block and a bitstream representation of the current video block using the regular merge candidate list.

In yet another example aspect, another method of video processing is disclosed. The method includes generating a candidate list by applying a pruning process to geometry predictor mode candidates using a rule that is based on where regular motion vectors from which the geometry predictor mode candidates are derived, and performing a conversion between a current video block and a bitstream representation of the current video block using the candidate list.

In yet another aspect, another method of video processing is disclosed. The method includes performing a determination, for a conversion between a current video block and a bitstream representation of the current video block, to disable use of a pruning operation for history based motion vector predictor table updating processes, wherein the determination is based on a video characteristic and performing the conversion based on the determination to disable use of the pruning operation.

In yet another example aspect, another method of video processing is disclosed. The method includes performing a determination of a maximum number of candidates allowed for at least one of (1) a geometry prediction mode merge candidate list for a current video block, or (2) a maximum number of base merge candidates in a motion vector difference (MMVD) merge candidate list, or (3) a maximum number of merge candidates in a sub-block based MMVD merge candidate list, or (4) a maximum number of affine merge candidates in a geometry prediction mode list, and performing, based on the determination, a conversion between the current video block and a bitstream representation of the current block, wherein the maximum number of candidates is signaled in an indicator in the bitstream representation.

In yet another example aspect, another method of video processing is disclosed. The method includes performing a determination of a maximum number of candidates allowed for at least one of (1) a geometry prediction mode merge candidate list for a current video block, or (2) a maximum number of base merge candidates in a motion vector difference (MMVD) merge candidate list, or (3) a maximum number of merge candidates in a sub-block based MMVD merge candidate list or (4) a maximum number of affine merge candidates in a geometry prediction mode list, and performing, based on the determination, a conversion between the current video block and a bitstream representation of the current block, wherein the maximum number of candidates allowed is determined to be equal to a maximum number of candidates in a regular merge candidate list.

In yet another example aspect, a video encoder apparatus is disclosed. The video encoder apparatus includes a processor that is configured to implement a method described herein.

In yet another example aspect, a video decoder apparatus is disclosed. The video decoder apparatus includes a processor that is configured to implement a method described herein.

In yet another aspect, a computer readable medium having code stored thereupon is disclosed. The code, when executed by a processor, causes the processor to implement a method described in the present document.

These, and other, aspects are described in the present document.

LISTING OF FIGURES

FIG. 1 shows an example of a derivation process for merge candidates list construction.

FIG. 2 shows example positions of spatial merge candidates.

FIG. 3 shows examples of candidate pairs considered for redundancy check of spatial merge candidates.

FIG. 4A-4B show example positions for the second PU of N×2N and 2N×N partitions.

FIG. 5 is an illustration of motion vector scaling for temporal merge candidate.

FIG. 6 shows candidate positions for temporal merge candidate, C0 and C1.

FIG. 7 shows example of combined bi-predictive merge candidate.

FIG. 8 summarizes derivation process for motion vector prediction candidate.

FIG. 9 is an example illustration of motion vector scaling for spatial motion vector candidate.

FIG. 10 shows an example of alternative motion vector predictor (ATMVP) motion prediction for a coding unit CU.

FIG. 11 shows example of one CU with four sub-blocks (A-D) and its neighbouring blocks (a-d).

FIG. 12 shows an example flowchart of encoding with different MV precision.

FIG. 13A shows a 135 degree partition type (splitting from top-left corner to bottom-right corner).

FIG. 13B shows a 45 degree splitting patterns.

FIG. 14 shows position of the neighboring blocks.

FIG. 15 shows an example of a CU applying the 1st weighting factor group.

FIG. 16A-16B show examples of motion vector storage.

FIG. 17 shows an example of neighboring blocks (A and L) used for context selection in TPM flag coding.

FIG. 18A-18B show respectively 4 and 6 parameter simplified affine motion models.

FIG. 19 shows an example of Affine MVF per sub-block.

FIG. 20A shows an example of a 4-parameter affine model.

FIG. 20B shows an example of a 6-parameter affine model.

FIG. 21 shows an example of an MVP for AF_INTER for inherited affine candidates.

FIG. 22 shows example MVP for AF_INTER for constructed affine candidates.

FIG. 23A shows an example of candidates for AF_MERGE in a five neighboring block scenario.

FIG. 23B shows an example flow of a CPMV predictor derivation process.

FIG. 24 shows example Candidates position for affine merge mode.

FIG. 25 shows an example of the intra block compensation.

FIG. 26 shows an example of a valid corresponding region in the collocated picture.

FIG. 27 shows an example coding flow for history based motion vector prediction (HMVP).

FIG. 28 depicts a modified merge candidate list construction process.

FIG. 29 shows an example of a UMVE search process.

FIG. 30 shows an example of a UMVE search point.

FIG. 31 is a block diagram of an example of a video processing apparatus.

FIG. 32 is a flowchart for an example of a video processing method.

FIG. 33 is a flowchart for another example of a video processing method.

FIG. 34 is a flowchart for an example of a video processing method.

DETAILED DESCRIPTION

The present document provides various techniques that can be used by a decoder of video bitstreams to improve the quality of decompressed or decoded digital video or images. Furthermore, a video encoder may also implement these techniques during the process of encoding in order to reconstruct decoded frames used for further encoding.

Section headings are used in the present document for ease of understanding and do not limit the embodiments and techniques to the corresponding sections. As such, embodiments from one section can be combined with embodiments from other sections.

1. Summary

This patent document is related to video coding technologies. Specifically, it is related to motion vector coding in video coding. It may be applied to the existing video coding standard like HEVC, or the standard (Versatile Video Coding) to be finalized. It may be also applicable to future video coding standards or video codec.

In the present document, the term “video processing” may refer to video encoding, video decoding, video compression or video decompression. For example, video compression algorithms may be applied during conversion from pixel representation of a video to a corresponding bitstream representation or vice versa.

2. Introductory Remarks

Video coding standards have evolved primarily through the development of the well-known ITU-T and ISO/IEC standards. The ITU-T produced H.261 and H.263, ISO/IEC produced MPEG-1 and MPEG-4 Visual, and the two organizations jointly produced the H.262/MPEG-2 Video and H.264/MPEG-4 Advanced Video Coding (AVC) and H.265/HEVC standards. Since H.262, the video coding standards are based on the hybrid video coding structure wherein temporal prediction plus transform coding are utilized. To explore the future video coding technologies beyond HEVC, Joint Video Exploration Team (JVET) was founded by VCEG and MPEG jointly in 2015. Since then, many new methods have been adopted by JVET and put into the reference software named Joint Exploration Model (JEM). In April 2018, the Joint Video Expert Team (JVET) between VCEG (Q6/16) and ISO/IEC JTC1 SC29/WG11 (MPEG) was created to work on the VVC standard targeting at 50% bitrate reduction compared to HEVC.

2.1 Inter Prediction in HEVC/H.265

Each inter-predicted PU has motion parameters for one or two reference picture lists. Motion parameters include a motion vector and a reference picture index. Usage of one of the two reference picture lists may also be signalled using inter_pred_idc. Motion vectors may be explicitly coded as deltas relative to predictors.

When a CU is coded with skip mode, one PU is associated with the CU, and there are no significant residual coefficients, no coded motion vector delta or reference picture index. A merge mode is specified whereby the motion parameters for the current PU are obtained from neighbouring PUs, including spatial and temporal candidates. The merge mode can be applied to any inter-predicted PU, not only for skip mode. The alternative to merge mode is the explicit transmission of motion parameters, where motion vector (to be more precise, motion vector differences (MVD) compared to a motion vector predictor), corresponding reference picture index for each reference picture list and reference picture list usage are signalled explicitly per each PU. Such a mode is named Advanced motion vector prediction (AMVP) in this disclosure.

When signalling indicates that one of the two reference picture lists is to be used, the PU is produced from one block of samples. This is referred to as ‘uni-prediction’. Uni-prediction is available both for P-slices and B-slices.

When signalling indicates that both of the reference picture lists are to be used, the PU is produced from two blocks of samples. This is referred to as ‘bi-prediction’. Bi-prediction is available for B-slices only.

The following text provides the details on the inter prediction modes specified in HEVC. The description will start with the merge mode.

2.1.1 Reference Picture List

In HEVC, the term inter prediction is used to denote prediction derived from data elements (e.g., sample values or motion vectors) of reference pictures other than the current decoded picture. Like in H.264/AVC, a picture can be predicted from multiple reference pictures. The reference pictures that are used for inter prediction are organized in one or more reference picture lists. The reference index identifies which of the reference pictures in the list should be used for creating the prediction signal.

A single reference picture list, List 0, is used for a P slice and two reference picture lists, List 0 and List 1 are used for B slices. It should be noted reference pictures included in List 0/1 could be from past and future pictures in terms of capturing/display order.

2.1.2 Merge Mode

2.1.2.1 Derivation of Candidates for Merge Mode

When a PU is predicted using merge mode, an index pointing to an entry in the merge candidates list is parsed from the bitstream and used to retrieve the motion information. The construction of this list is specified in the HEVC standard and can be summarized according to the following sequence of steps:

-   -   Step 1: Initial candidates derivation         -   Step 1.1: Spatial candidates derivation         -   Step 1.2: Redundancy check for spatial candidates         -   Step 1.3: Temporal candidates derivation     -   Step 2: Virtual candidates insertion         -   Step 2.1: Creation of combined bi-predictive candidates         -   Step 2.2: Insertion of default motion candidates (zero             motion candidates)

These steps are also schematically depicted in FIG. 1. For spatial merge candidate derivation, a maximum of four merge candidates are selected among candidates that are located in five different positions. For temporal merge candidate derivation, a maximum of one merge candidate is selected among two candidates. Since constant number of candidates for each PU is assumed at decoder, additional candidates are generated when the number of candidates obtained from step 1 does not reach the maximum number of merge candidate (MaxNumMergeCand) which is signalled in slice header. Since the number of candidates is constant, index of best merge candidate is encoded using truncated unary binarization (TU). If the size of CU is equal to 8, all the PUs of the current CU share a single merge candidate list, which is identical to the merge candidate list of the 2N×2N prediction unit.

In the following, the operations associated with the aforementioned steps are detailed.

FIG. 1 shows an example of a derivation process for merge candidates list construction.

2.1.2.2 Spatial Candidates Derivation

In the derivation of spatial merge candidates, a maximum of four merge candidates are selected among candidates located in the positions depicted in FIG. 2.

FIG. 2 shows example positions of spatial merge candidates.

The order of derivation is A₁, B₁, B₀, A₀ and B₂. Position B₂ is considered only when any PU of position A₁, B₁, B₀, A₀ is not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position A₁ is added, the addition of the remaining candidates is subject to a redundancy check which ensures that candidates with same motion information are excluded from the list so that coding efficiency is improved.

FIG. 3 shows examples of candidate pairs considered for redundancy check of spatial merge candidates. To reduce computational complexity, not all possible candidate pairs are considered in the mentioned redundancy check. Instead only the pairs linked with an arrow in FIG. 3 are considered and a candidate is only added to the list if the corresponding candidate used for redundancy check has not the same motion information. Another source of duplicate motion information is the “second PU” associated with partitions different from 2N×2N. As an example, FIG. 4A-4B depict the second PU for the case of N×2N and 2N×N, respectively. When the current PU is partitioned as N×2N, candidate at position A1 is not considered for list construction. In fact, by adding this candidate will lead to two prediction units having the same motion information, which is redundant to just have one PU in a coding unit. Similarly, position B1 is not considered when the current PU is partitioned as 2N×N.

FIG. 4A-4B show example positions for the second PU of N×2N and 2N×N partitions.

2.1.2.3 Temporal Candidates Derivation

In this step, only one candidate is added to the list. Particularly, in the derivation of this temporal merge candidate, a scaled motion vector is derived based on co-located PU belonging to the picture which has the smallest POC difference with current picture within the given reference picture list. The reference picture list to be used for derivation of the co-located PU is explicitly signalled in the slice header. The scaled motion vector for temporal merge candidate is obtained as illustrated by the dotted line in FIG. 5, which is scaled from the motion vector of the co-located PU using the POC distances, tb and td, where tb is defined to be the POC difference between the reference picture of the current picture and the current picture and td is defined to be the POC difference between the reference picture of the co-located picture and the co-located picture. The reference picture index of temporal merge candidate is set equal to zero. A practical realization of the scaling process is described in the HEVC specification. For a B-slice, two motion vectors, one is for reference picture list 0 and the other is for reference picture list 1, are obtained and combined to make the bi-predictive merge candidate.

FIG. 5 is an illustration of motion vector scaling for temporal merge candidate.

In the co-located PU (Y) belonging to the reference frame, the position for the temporal candidate is selected between candidates C0 and C1, as depicted FIG. 6. If PU at position C0 is not available, is intra coded, or is outside of the current coding tree unit (CTU aka. LCU, largest coding unit) row, position C1 is used. Otherwise, position C0 is used in the derivation of the temporal merge candidate.

FIG. 6 shows candidate positions for temporal merge candidate, C0 and C1.

2.1.2.4 Virtual Candidates Insertion

Besides spatial and temporal merge candidates, there are two additional types of virtual merge candidates: combined bi-predictive merge candidate and zero merge candidate.

2.1.2.4.1 Combined Bi-Predictive Merge Candidates

Combined bi-predictive merge candidates are generated by utilizing spatial and temporal merge candidates. Combined bi-predictive merge candidate is used for B-Slice only. The combined bi-predictive candidates are generated by combining the first reference picture list motion parameters of an initial candidate with the second reference picture list motion parameters of another. If these two tuples provide different motion hypotheses, they will form a new bi-predictive candidate.

FIG. 7 shows example of combined bi-predictive merge candidate.

As an example, FIG. 7 depicts the case when two candidates in the original list (on the left), which have mvL0 and refIdxL0 or mvL1 and refIdxL1, are used to create a combined bi-predictive merge candidate added to the final list (on the right). There are numerous rules regarding the combinations which are considered to generate these additional merge candidates.

2.1.2.4.2 Default Motion Candidates

Zero motion candidates are inserted to fill the remaining entries in the merge candidates list and therefore hit the MaxNumMergeCand capacity. These candidates have zero spatial displacement and a reference picture index which starts from zero and increases every time a new zero motion candidate is added to the list.

More specifically, the following steps are performed in order till the merge list is full:

-   -   1. Set variable numRef to either number of reference picture         associated with list 0 for P slice, or the minimum number of         reference pictures in two lists for B slice;     -   2. Add non-repeated zero motion candidates:         -   For variable i being 0 . . . numRef−1, add a default motion             candidate with MV set to (0, 0) and reference picture index             set to i for list 0 (if P slice), or for both lists (if B             slice).     -   3. Add repeated zero motion candidates with MV set to (0, 0),         reference picture index of list 0 set to 0 (if P slice) and         reference picture indices of both lists set to 0 (if B slice).

2.1.3 AMVP

AMVP exploits spatio-temporal correlation of motion vector with neighbouring PUs, which is used for explicit transmission of motion parameters. For each reference picture list, a motion vector candidate list is constructed by firstly checking availability of left, above temporally neighbouring PU positions, removing redundant candidates and adding zero vector to make the candidate list to be constant length. Then, the encoder can select the best predictor from the candidate list and transmit the corresponding index indicating the chosen candidate. Similarly with merge index signalling, the index of the best motion vector candidate is encoded using truncated unary. The maximum value to be encoded in this case is 2 (see FIG. 8). In the following sections, details about derivation process of motion vector prediction candidate are provided.

2.1.3.1 Derivation of AMVP Candidates

FIG. 8 summarizes derivation process for motion vector prediction candidate.

In motion vector prediction, two types of motion vector candidates are considered: spatial motion vector candidate and temporal motion vector candidate. For spatial motion vector candidate derivation, two motion vector candidates are eventually derived based on motion vectors of each PU located in five different positions as depicted in FIG. 2.

For temporal motion vector candidate derivation, one motion vector candidate is selected from two candidates, which are derived based on two different co-located positions. After the first list of spatio-temporal candidates is made, duplicated motion vector candidates in the list are removed. If the number of potential candidates is larger than two, motion vector candidates whose reference picture index within the associated reference picture list is larger than 1 are removed from the list. If the number of spatio-temporal motion vector candidates is smaller than two, additional zero motion vector candidates (with MV set to (0, 0)) is added to the list.

2.1.3.2 Spatial Motion Vector Candidates

In the derivation of spatial motion vector candidates, a maximum of two candidates are considered among five potential candidates, which are derived from PUs located in positions as depicted in FIG. 2, those positions being the same as those of motion merge. The order of derivation for the left side of the current PU is defined as A₀, A₁, and scaled A₀,scaled A₁. The order of derivation for the above side of the current PU is defined as B₀, B₁, B₂, scaled B₀, scaled B₁, scaled B₂. For each side there are therefore four cases that can be used as motion vector candidate, with two cases not required to use spatial scaling, and two cases where spatial scaling is used. The four different cases are summarized as follows.

-   -   No spatial scaling         -   (1) Same reference picture list, and same reference picture             index (same POC)         -   (2) Different reference picture list, but same reference             picture (same POC)     -   Spatial scaling         -   (3) Same reference picture list, but different reference             picture (different POC)         -   (4) Different reference picture list, and different             reference picture (different POC)

The no-spatial-scaling cases are checked first followed by the spatial scaling. Spatial scaling is considered when the POC is different between the reference picture of the neighbouring PU and that of the current PU regardless of reference picture list. If all PUs of left candidates are not available or are intra coded, scaling for the above motion vector is allowed to help parallel derivation of left and above MV candidates. Otherwise, spatial scaling is not allowed for the above motion vector.

FIG. 9 is an example illustration of motion vector scaling for spatial motion vector candidate.

In a spatial scaling process, the motion vector of the neighbouring PU is scaled in a similar manner as for temporal scaling, as depicted as FIG. 9. The main difference is that the reference picture list and index of current PU is given as input; the actual scaling process is the same as that of temporal scaling.

2.1.3.3 Temporal Motion Vector Candidates

Apart for the reference picture index derivation, all processes for the derivation of temporal merge candidates are the same as for the derivation of spatial motion vector candidates (see FIG. 6). The reference picture index is signalled to the decoder.

2.2 Sub-CU Based Motion Vector Prediction Methods in JEM

In the JEM with QTBT, each CU can have at most one set of motion parameters for each prediction direction. Two sub-CU level motion vector prediction methods are considered in the encoder by splitting a large CU into sub-CUs and deriving motion information for all the sub-CUs of the large CU. Alternative temporal motion vector prediction (ATMVP) method allows each CU to fetch multiple sets of motion information from multiple blocks smaller than the current CU in the collocated reference picture. In spatial-temporal motion vector prediction (STMVP) method motion vectors of the sub-CUs are derived recursively by using the temporal motion vector predictor and spatial neighbouring motion vector.

To preserve more accurate motion field for sub-CU motion prediction, the motion compression for the reference frames is currently disabled.

FIG. 10 shows an example of ATMVP motion prediction for a CU.

2.2.1 Alternative Temporal Motion Vector Prediction

In the alternative temporal motion vector prediction (ATMVP) method, the motion vectors temporal motion vector prediction (TMVP) is modified by fetching multiple sets of motion information (including motion vectors and reference indices) from blocks smaller than the current CU. In an example, the sub-CUs are square N×N blocks (N is set to 4 by default).

ATMVP predicts the motion vectors of the sub-CUs within a CU in two steps. The first step is to identify the corresponding block in a reference picture with a so-called temporal vector. The reference picture is called the motion source picture. The second step is to split the current CU into sub-CUs and obtain the motion vectors as well as the reference indices of each sub-CU from the block corresponding to each sub-CU.

In the first step, a reference picture and the corresponding block is determined by the motion information of the spatial neighbouring blocks of the current CU. To avoid the repetitive scanning process of neighbouring blocks, the first merge candidate in the merge candidate list of the current CU is used. The first available motion vector as well as its associated reference index are set to be the temporal vector and the index to the motion source picture. This way, in ATMVP, the corresponding block may be more accurately identified, compared with TMVP, wherein the corresponding block (sometimes called collocated block) is always in a bottom-right or center position relative to the current CU.

In the second step, a corresponding block of the sub-CU is identified by the temporal vector in the motion source picture, by adding to the coordinate of the current CU the temporal vector. For each sub-CU, the motion information of its corresponding block (the smallest motion grid that covers the center sample) is used to derive the motion information for the sub-CU. After the motion information of a corresponding N×N block is identified, it is converted to the motion vectors and reference indices of the current sub-CU, in the same way as TMVP of HEVC, wherein motion scaling and other procedures apply. For example, the decoder checks whether the low-delay condition (i.e. the POCs of all reference pictures of the current picture are smaller than the POC of the current picture) is fulfilled and possibly uses motion vector MV_(x) (the motion vector corresponding to reference picture list X) to predict motion vector MV_(y) (with X being equal to 0 or 1 and Y being equal to 1−X) for each sub-CU.

2.2.2 Spatial-Temporal Motion Vector Prediction (STMVP)

In this method, the motion vectors of the sub-CUs are derived recursively, following raster scan order. FIG. 11 illustrates this concept. Let us consider an 8×8 CU which contains four 4×4 sub-CUs A, B, C, and D. The neighbouring 4×4 blocks in the current frame are labelled as a, b, c, and d.

The motion derivation for sub-CU A starts by identifying its two spatial neighbours. The first neighbour is the N×N block above sub-CU A (block c). If this block c is not available or is intra coded the other N×N blocks above sub-CU A are checked (from left to right, starting at block c). The second neighbour is a block to the left of the sub-CU A (block b). If block b is not available or is intra coded other blocks to the left of sub-CU A are checked (from top to bottom, staring at block b). The motion information obtained from the neighbouring blocks for each list is scaled to the first reference frame for a given list. Next, temporal motion vector predictor (TMVP) of sub-block A is derived by following the same procedure of TMVP derivation as specified in HEVC. The motion information of the collocated block at location D is fetched and scaled accordingly. Finally, after retrieving and scaling the motion information, all available motion vectors (up to 3) are averaged separately for each reference list. The averaged motion vector is assigned as the motion vector of the current sub-CU.

FIG. 11 shows example of one CU with four sub-blocks (A-D) and its neighbouring blocks (a-d).

2.2.3 Sub-CU Motion Prediction Mode Signalling

The sub-CU modes are enabled as additional merge candidates and there is no additional syntax element required to signal the modes. Two additional merge candidates are added to merge candidates list of each CU to represent the ATMVP mode and STMVP mode. Up to seven merge candidates are used, if the sequence parameter set indicates that ATMVP and STMVP are enabled. The encoding logic of the additional merge candidates is the same as for the merge candidates in the HM, which means, for each CU in P or B slice, two more RD checks is needed for the two additional merge candidates.

In the JEM, all bins of merge index is context coded by CABAC. While in HEVC, only the first bin is context coded and the remaining bins are context by-pass coded.

2.3 Inter Prediction Methods in VVC

There are several new coding tools for inter prediction improvement, such as Adaptive motion vector difference resolution (AMVR) for signaling MVD, affine prediction mode, Triangular prediction mode (TPM), multiple hypothesis intra mode (MHIntra, a.k.a., intra-inter)ATMVP, Generalized Bi-Prediction (GBI), Bi-directional Optical flow (BIO).

2.3.1 Adaptive Motion Vector Difference Resolution

In HEVC, motion vector differences (MVDs) (between the motion vector and predicted motion vector of a PU) are signalled in units of quarter luma samples when use_integer_mv_flag is equal to 0 in the slice header. In the VVC, a locally adaptive motion vector resolution (LAMVR) is introduced. In the VVC, MVD can be coded in units of quarter luma samples, integer luma samples or four luma samples (i.e., 1/4-pel, 1-pel, 4-pel). The MVD resolution is controlled at the coding unit (CU) level, and MVD resolution flags are conditionally signalled for each CU that has at least one non-zero MVD components.

For a CU that has at least one non-zero MVD components, a first flag is signalled to indicate whether quarter luma sample MV precision is used in the CU. When the first flag (equal to 1) indicates that quarter luma sample MV precision is not used, another flag is signalled to indicate whether integer luma sample MV precision or four luma sample MV precision is used.

When the first MVD resolution flag of a CU is zero, or not coded for a CU (meaning all MVDs in the CU are zero), the quarter luma sample MV resolution is used for the CU. When a CU uses integer-luma sample MV precision or four-luma-sample MV precision, the MVPs in the AMVP candidate list for the CU are rounded to the corresponding precision.

In the encoder, CU-level RD checks are used to determine which MVD resolution is to be used for a CU. That is, the CU-level RD check is performed three times for each MVD resolution. To accelerate encoder speed, the following encoding schemes are applied in the JEM.

-   -   During RD check of a CU with normal quarter luma sample MVD         resolution, the motion information of the current CU (integer         luma sample accuracy) is stored. The stored motion information         (after rounding) is used as the starting point for further small         range motion vector refinement during the RD check for the same         CU with integer luma sample and 4 luma sample MVD resolution so         that the time-consuming motion estimation process is not         duplicated three times.     -   RD check of a CU with 4 luma sample MVD resolution is         conditionally invoked. For a CU, when RD cost integer luma         sample MVD resolution is much larger than that of quarter luma         sample MVD resolution, the RD check of 4 luma sample MVD         resolution for the CU is skipped.

The encoding process is shown in FIG. 12. First, 1/4 pel MV is tested and the RD cost is calculated and denoted as RDCost0, then integer MV is tested and the RD cost is denoted as RDCost1. If RDCost1<th*RDCost0 (wherein th is a positive value), then 4-pel MV is tested; otherwise, 4-pel MV is skipped. Basically, motion information and RD cost etc. are already known for 1/4 pel MV when checking integer or 4-pel MV, which can be reused to speed up the encoding process of integer or 4-pel MV.

FIG. 12 shows an example flowchart of encoding with different MV precision.

2.3.2 Triangular Prediction Mode

The concept of the triangular prediction mode (TPM) is to introduce a new triangular partition for motion compensated prediction. As shown in FIG. 13A-13B, it splits a CU into two triangular prediction units, in either diagonal or inverse diagonal direction. Each triangular prediction unit in the CU is inter-predicted using its own uni-prediction motion vector and reference frame index which are derived from a single uni-prediction candidate list. An adaptive weighting process is performed to the diagonal edge after predicting the triangular prediction units. Then, the transform and quantization process are applied to the whole CU. It is noted that this mode is only applied to merge mode (note: skip mode is treated as a special merge mode).

FIG. 13 A shows a 135 degree partition type (splitting from top-left corner to bottom-right corner). FIG. 13B shows a 45 degree splitting patterns.

2.3.2.1 Uni-Prediction Candidate List for TPM

FIG. 14 shows position of the neighboring blocks. This section, and other discussion in the present document with respect to TPM, is generally applicable to any geometry partition mode, and is specifically illustrated with reference to TPM mode.

The uni-prediction candidate list, named TPM motion candidate list, consists of five uni-prediction motion vector candidates. It is derived from seven neighboring blocks including five spatial neighboring blocks (1 to 5) and two temporal co-located blocks (6 to 7), as shown in FIG. 14. The motion vectors of the seven neighboring blocks are collected and put into the uni-prediction candidate list according in the order of uni-prediction motion vectors, L0 motion vector of bi-prediction motion vectors, L1 motion vector of bi-prediction motion vectors, and averaged motion vector of the L0 and L1 motion vectors of bi-prediction motion vectors. If the number of candidates is less than five, zero motion vector is added to the list. Motion candidates added in this list for TPM are called TPM candidates, motion information derived from spatial/temporal blocks are called regular motion candidates.

More specifically, the following steps are involved:

-   -   1) Obtain regular motion candidates from A₁, B₁, B₀, A₀, B₂, Col         and Col2 (corresponding to block 1-7 in FIG. 14) with full         pruning operations when adding a regular motion candidate from         spatial neighboring blocks.     -   2) Set variable numCurrMrgCand=0     -   3) For each regular motion candidates derived from A₁, B₁, B₀,         A₀, B₂, Col and Col2, if not pruned and numCurrMrgCand is less         than 5, if the regular motion candidate is uni-prediction         (either from List 0 or List 1), it is directly added to the         merge list as an TPM candidate with numCurrMrgCand increased         by 1. Such a TPM candidate is named ‘originally uni-predicted         candidate’.         -   Full pruning is applied.     -   4) For each motion candidates derived from A₁, B₁, B₀, A₀, B₂,         Col and Col2, if not pruned, and numCurrMrgCand is less than 5,         if the regular motion candidate is bi-prediction, the motion         information from List 0 is added to the TPM merge list (that is,         modified to be uni-prediction from List 0) as a new TPM         candidate and numCurrMrgCand increased by 1. Such a TPM         candidate is named ‘Truncated List0-predicted candidate’.         -   Full pruning is applied.     -   5) For each motion candidates derived from A₁, B₁, B₀, A₀, B₂,         Col and Col2, if not pruned, and numCurrMrgCand is less than 5,         if the regular motion candidate is bi-prediction, the motion         information from List 1 is added to the TPM merge list (that is,         modified to be uni-prediction from List 1) and numCurrMrgCand         increased by 1. Such a TPM candidate is named ‘Truncated         List1-predicted candidate’.         -   Full pruning is applied.     -   6) For each motion candidates derived from A₁, B₁, B₀, A₀, B₂,         Col and Col2, if not pruned, and numCurrMrgCand is less than 5,         if the regular motion candidate is bi-prediction,         -   If List 0 reference picture's slice QP is smaller than List             1 reference picture's slice QP, the motion information of             List 1 is firstly scaled to List 0 reference picture, and             the average of the two MVs (one is from original List 0, and             the other is the scaled MV from List 1) is added to the TPM             merge list, such a candidate is called averaged             uni-prediction from List 0 motion candidate and             numCurrMrgCand increased by 1.         -   Otherwise, the motion information of List 0 is firstly             scaled to List 1 reference picture, and the average of the             two MVs (one is from original List 1, and the other is the             scaled MV from List 0) is added to the TPM merge list, such             a TPM candidate is called averaged uni-prediction from List             1 motion candidate and numCurrMrgCand increased by 1.         -   Full pruning is applied.     -   7) If numCurrMrgCand is less than 5, zero motion vector         candidates are added without pruning.         -   a. Set variable numRef to either number of reference picture             associated with list 0 for P slice, or the minimum number of             reference pictures in two lists for B slice;         -   b. Add non-repeated zero motion candidates:             -   i. For variable i being 0 . . . numRef−1, add two TPM                 candidates in order:                 -   a default motion candidate with MV set to (0, 0) and                     reference picture index set to i for list 0                 -   a default motion candidate with MV set to (0, 0) and                     reference picture index set to i for list 1

When inserting a candidate to the list, if it has to be compared to all previously added candidates to see whether it is identical to one of them, such a process is called full pruning.

2.3.2.2 Adaptive Weighting Process

After predicting each triangular prediction unit, an adaptive weighting process is applied to the diagonal edge between the two triangular prediction units to derive the final prediction for the whole CU. Two weighting factor groups are defined as follows:

-   -   1^(st) weighting factor group: {7/8, 6/8, 4/8, 2/8, 1/8} and         {7/8, 4/8, 1/8} are used for the luminance and the chrominance         samples, respectively;     -   2^(nd) weighting factor group: {7/8, 6/8, 5/8, 4/8, 3/8, 2/8,         1/8} and {6/8, 4/8, 2/8} are used for the luminance and the         chrominance samples, respectively.

Weighting factor group is selected based on the comparison of the motion vectors of two triangular prediction units. The 2^(nd) weighting factor group is used when any one of the following condition is true:

-   -   the reference pictures of the two triangular prediction units         are different from each other     -   absolute value of the difference of two motion vectors'         horizontal values is larger than 16 pixels.     -   absolute value of the difference of two motion vectors' vertical         values is larger than 16 pixels.

Otherwise, the 1st weighting factor group is used. An example is shown in FIG. 15.

FIG. 15 shows an example of a CU applying the 1st weighting factor group.

2.3.2.3 Motion Vector Storage

The motion vectors (Mv1 and Mv2 in FIGS. 16A-16B) of the triangular prediction units are stored in 4×4 grids. For each 4×4 grid, either uni-prediction or bi-prediction motion vector is stored depending on the position of the 4×4 grid in the CU. As shown in FIG. 16A-16B, uni-prediction motion vector, either Mv1 or Mv2, is stored for the 4×4 grid located in the non-weighted area (that is, not located at the diagonal edge). On the other hand, a bi-prediction motion vector is stored for the 4×4 grid located in the weighted area. The bi-prediction motion vector is derived from Mv1 and Mv2 according to the following rules:

-   -   1) In the case that Mv1 and Mv2 have motion vector from         different directions (L0 or L1), Mv1 and Mv2 are simply combined         to form the bi-prediction motion vector.     -   2) In the case that both Mv1 and Mv2 are from the same L0 (or         L1) direction,         -   If the reference picture of Mv2 is the same as a picture in             the L1 (or L0) reference picture list, Mv2 is scaled to the             picture. Mv1 and the scaled Mv2 are combined to form the             bi-prediction motion vector.         -   If the reference picture of Mv1 is the same as a picture in             the L1 (or L0) reference picture list, Mv1 is scaled to the             picture. The scaled Mv1 and Mv2 are combined to form the             bi-prediction motion vector.         -   Otherwise, only Mv1 is stored for the weighted area.

FIG. 16A-16B show examples of motion vector storage.

2.3.2.4 Signaling of Triangular Prediction Mode (TPM)

One bit flag to indicate whether TPM is used may be firstly signaled at CU level. Afterwards, the indications of two splitting patterns (as depicted in FIGS. 13A-13B), and selected merge indices for each of the two partitions are further signaled.

When any one of the following condition is true for a CU, signaling of the flag is skipped and TPM is not applied.

-   -   If the SPS flag of TPM usage is false     -   If the slice type of the slice covering current block is not a B         slice     -   If the current block size is smaller than 64     -   If the current block is coded with affine mode

2.3.2.4.1 Signaling of TPM Flag

Let's denote one luma block's width and height by W and H, respectively. If W*H<64, triangular prediction mode is disabled.

When one block is coded with affine mode, triangular prediction mode is also disabled.

When one block is coded with merge mode, one bit flag may be signaled to indicate whether the triangular prediction mode is enabled or disabled for the block.

The flag is coded with 3 contexts, based on the following equation: Ctx index=((left block L available && L is coded with TPM?)1: 0)+((Above block A available && A is coded with TPM?)1: 0);

FIG. 17 shows an example of neighboring blocks (A and L) used for context selection in TPM flag coding.

2.3.2.4.2 Signaling of an Indication of Two Splitting Patterns (as Depicted in FIGS. 13A-13B), and Selected Merge Indices for Each of the Two Partitions

It is noted that splitting patterns, merge indices of two partitions are jointly coded. In an example, it is restricted that the two partitions couldn't use the same reference index. Therefore, there are 2 (splitting patterns)*N (maximum number of merge candidates)*(N−1) possibilities wherein N is set to 5. One indication is coded and the mapping between the splitting patterns, two merge indices and coded indication are derived from the array defined below:

const  unint  8_t  g_Triangle Combination [TRIANGLE_MAX_NUM_CANDS][3] = {{θ, 1, θ}, {1, θ, 1}, {1, θ, 2}, {θ, θ, 1}, {θ, 2, θ}{1, θ, 3}, {1, θ, 4}, {1, 1, θ}, {θ, 3, θ}, {θ, 4, θ}{θ, θ, 2}, {θ, 1, 2}, {1, 1, 2}, {θ, θ, 4}, {θ, θ, 3}, {θ, 1, 3}, {θ, 1, 4}{1, 1, 3}{1, 2, 1}{1, 2, θ}, {θ, 2, 1}, {θ, 4, 3}, {1, 3, θ}, {1, 3, 2}, {1, 3, 4}, {1, 4, θ}, {1, 3, 1}, {1, 2, 3}{1, 4, 1}, {θ, 4, 1}{θ, 2, 3}, {1, 4, 2}, {θ, 3, 2}, {1, 4, 3}, {θ, 3, 1}, {θ, 2, 4}, {1, 2, 4}, {θ, 4, 2}, {0, 3, 4}};

splitting patterns (45 degree or 135 degree)=g_TriangleCombination[signaled indication][0];

Merge index of candidate A=g_TriangleCombination[signaled indication][1];

Merge index of candidate B=g_TriangleCombination[signaled indication][2];

Once the two motion candidates A and B are derived, the two partitions' (PU1 and PU2) motion information could be set either from A or B. Whether PU1 uses the motion information of merge candidate A or B is dependent on the prediction directions of the two motion candidates. Table 1 shows the relationship between two derived motion candidates A and B, with the two partitions.

TABLE 1 Derivation of partitions' motion information from derived two merge candidates (A, B) Prediction Prediction PU1s motion PU2' s motion direction of A direction of B information information L0 L0 A (L0) B (L0) L1 L1 B (L1) A (L1) L0 L1 A (L0) B (L1) L1 L0 B (L0) A (L1)

2.3.2.4.3 Entropy Coding of the Indication (Denoted by Merge_Triangle_Idx)

merge_triangle_idx is within the range [0, 39], inclusively. K-th order Exponential Golomb (EG) code is used for binarization of merge_triangle_idx wherein K is set to 1.

K-th Order EG

To encode larger numbers in fewer bits (at the expense of using more bits to encode smaller numbers), this can be generalized using a nonnegative integer parameter k. To encode a nonnegative integer x in an order-k exp-Golomb code:

1. Encode └x/2^(k)┘ using order-0 exp-Golomb code described above, then

2. Encode x mod 2^(k) in binary

TABLE 2 Exp-Golomb-k coding examples x k = 0 k = 1 k = 2 0 1 10 100 1 010 11 101 2 011 0100 110 3 00100 0101 111 4 00101 0110 01000 5 00110 0111 01001 6 00111 001000 01010 7 0001000 001001 01011 8 0001001 001010 01100 9 0001010 001011 01101 10 0001011 001100 01110 11 0001100 001101 01111 12 0001101 001110 0010000 13 0001110 001111 0010001 14 0001111 00010000 0010010 15 000010000 00010001 0010011 16 000010001 00010010 0010100 17 000010010 00010011 0010101 18 000010011 00010100 0010110 19 000010100 00010101 0010111

2.3.3 Affine Motion Compensation Prediction

In HEVC, only translation motion model is applied for motion compensation prediction (MCP). While in the real world, there are many kinds of motion, e.g. zoom in/out, rotation, perspective motions and the other irregular motions. In VVC, a simplified affine transform motion compensation prediction is applied with 4-parameter affine model and 6-parameter affine model. As shown FIG. 18A-18B, the affine motion field of the block is described by two control point motion vectors (CPMVs) for the 4-parameter affine model and 3 CPMVs for the 6-parameter affine model.

FIG. 18A-18B show respectively 4 and 6 parameter simplified affine motion models.

The motion vector field (MVF) of a block is described by the following equations with the 4-parameter affine model (wherein the 4-parameter are defined as the variables a, b, e and f) in equation (1) and 6-parameter affine model (wherein the 4-parameter are defined as the variables a, b, c, d, e and f) in equation (2) respectively:

$\begin{matrix} \left\{ \begin{matrix} {{{mv}^{h}\left( {x,y} \right)} = {{{ax} - {by} + e} = {{\frac{\left( {{mv_{1}^{h}} - {mv_{0}^{h}}} \right)}{w}x} - {\frac{\left( {{mv_{1}^{v}} - {mv_{0}^{v}}} \right)}{w}y} + {mv_{0}^{h}}}}} \\ {{{mv}^{v}\left( {x,y} \right)} = {{{bx} + {ay} + f} = {{\frac{\left( {{mv_{1}^{v}} - {mv_{0}^{v}}} \right)}{w}x} + {\frac{\left( {{mv_{1}^{h}} - {mv_{0}^{h}}} \right)}{w}y} + {mv_{0}^{v}}}}} \end{matrix} \right. & (1) \\ \left\{ \begin{matrix} {{m{v^{h}\left( {x,\ y} \right)}} = {{{ax} + {cy} + e} = {{\frac{\left( {{mv_{1}^{h}} - {mv_{0}^{h}}} \right)}{w}x} + {\frac{\left( {{mv_{2}^{h}} - {mv_{0}^{h}}} \right)}{h}y} + {mv_{0}^{h}}}}} \\ {{m{v^{v}\left( {x,y} \right)}} = {{{bx} + {dy} + f} = {{\frac{\left( {{mv_{1}^{v}} - {mv_{0}^{v}}} \right)}{w}x} + {\frac{\left( {{mv_{2}^{v}} - {mv_{0}^{v}}} \right)}{h}y} + {mv_{0}^{v}}}}} \end{matrix} \right. & (2) \end{matrix}$

where (mv^(h) ₀, mv^(h) ₀) is motion vector of the top-left corner control point, and (mv^(h) ₁, mv^(h) ₁) is motion vector of the top-right corner control point and (mv^(h) ₂, mv^(h) ₂) is motion vector of the bottom-left corner control point, all of the three motion vectors are called control point motion vectors (CPMV), (x, y) represents the coordinate of a representative point relative to the top-left sample within current block and (mv^(h)(x,y), mv^(v)(x,y)) is the motion vector derived for a sample located at (x, y). The CP motion vectors may be signaled (like in the affine AMVP mode) or derived on-the-fly (like in the affine merge mode). w and h are the width and height of the current block. In practice, the division is implemented by right-shift with a rounding operation. In VTM, the representative point is defined to be the center position of a sub-block, e.g., when the coordinate of the left-top corner of a sub-block relative to the top-left sample within current block is (xs, ys), the coordinate of the representative point is defined to be (xs+2, ys+2). For each sub-block (i.e., 4×4 in VTM), the representative point is utilized to derive the motion vector for the whole sub-block.

In order to further simplify the motion compensation prediction, sub-block based affine transform prediction is applied. To derive motion vector of each M×N (both M and N are set to 4 in current VVC) sub-block, the motion vector of the center sample of each sub-block, as shown in FIG. 19, is calculated according to Equation (1) and (2), and rounded to 1/16 fraction accuracy. Then the motion compensation interpolation filters for 1/16-pel are applied to generate the prediction of each sub-block with derived motion vector. The interpolation filters for 1/16-pel are introduced by the affine mode.

FIG. 19 shows an example of Affine MVF per sub-block.

After MCP, the high accuracy motion vector of each sub-block is rounded and saved as the same accuracy as the normal motion vector.

2.3.3.1 Signaling of Affine Prediction

Similar to the translational motion model, there are also two modes for signaling the side information due affine prediction. They are AFFINE_INTER and AFFINE_MERGE modes.

2.3.3.2 AF_INTER Mode

For CUs with both width and height larger than 8, AF_INTER mode can be applied. An affine flag in CU level is signalled in the bitstream to indicate whether AF_INTER mode is used.

In this mode, for each reference picture list (List 0 or List 1), an affine AMVP candidate list is constructed with three types of affine motion predictors in the following order, wherein each candidate includes the estimated CPMVs of the current block. The differences of the best CPMVs found at the encoder side (such as mv₀ mv₁ mv₂ in FIG. 22) and the estimated CPMVs are signalled. In addition, the index of affine AMVP candidate from which the estimated CPMVs are derived is further signalled.

1) Inherited Affine Motion Predictors

The checking order is similar to that of spatial MVPs in HEVC AMVP list construction. First, a left inherited affine motion predictor is derived from the first block in {A1, A0} that is affine coded and has the same reference picture as in current block. Second, an above inherited affine motion predictor is derived from the first block in {B1, B0, B2} that is affine coded and has the same reference picture as in current block. The five blocks A1, A0, B1, B0, B2 are depicted in FIG. 21.

Once a neighboring block is found to be coded with affine mode, the CPMVs of the coding unit covering the neighboring block are used to derive predictors of CPMVs of current block. For example, if A1 is coded with non-affine mode and A0 is coded with 4-parameter affine mode, the left inherited affine MV predictor will be derived from A0. In this case, the CPMVs of a CU covering A0, as denoted by MV₀ ^(N) for the top-left CPMV and MV₁ ^(N) for the top-right CPMV in FIG. 23B are utilized to derive the estimated CPMVs of current block, denoted by MV₀ ^(C), MV₁ ^(C), MV₂ ^(C) for the top-left (with coordinate (x0, y0)), top-right (with coordinate (x1, y1)) and bottom-right positions (with coordinate (x2, y2)) of current block.

2) Constructed Affine Motion Predictors

A constructed affine motion predictor consists of control-point motion vectors (CPMVs) that are derived from neighboring inter coded blocks, as shown in FIG. 22, that have the same reference picture. If the current affine motion model is 4-parameter affine, the number of CPMVs is 2, otherwise if the current affine motion model is 6-parameter affine, the number of CPMVs is 3. The top-left CPMV mv ₀ is derived by the MV at the first block in the group {A, B, C} that is inter coded and has the same reference picture as in current block. The top-right CPMV mv ₁ is derived by the MV at the first block in the group {D, E} that is inter coded and has the same reference picture as in current block. The bottom-left CPMV mv ₂ is derived by the MV at the first block in the group {F, G} that is inter coded and has the same reference picture as in current block.

-   -   If the current affine motion model is 4-parameter affine, then a         constructed affine motion predictor is inserted into the         candidate list only if both mv ₀ and mv ₁ are founded, that is,         mv ₀ and mv ₁ are used as the estimated CPMVs for top-left (with         coordinate (x0, y0)), top-right (with coordinate (x1, y1))         positions of current block.     -   If the current affine motion model is 6-parameter affine, then a         constructed affine motion predictor is inserted into the         candidate list only if mv ₀, mv ₁ and mv ₂ are all founded, that         is, mv ₀, mv ₁ and mv ₂ are used as the estimated CPMVs for         top-left (with coordinate (x0, y0)), top-right (with coordinate         (x1, y1)) and bottom-right (with coordinate (x2, y2)) positions         of current block.

No pruning process is applied when inserting a constructed affine motion predictor into the candidate list.

3) Normal AMVP Motion Predictors

The following applies until the number of affine motion predictors reaches the maximum.

1) Derive an affine motion predictor by setting all CPMVs equal to mv ₂ if available.

2) Derive an affine motion predictor by setting all CPMVs equal to mv ₁ if available.

3) Derive an affine motion predictor by setting all CPMVs equal to mv ₀ if available.

4) Derive an affine motion predictor by setting all CPMVs equal to HEVC TMVP if available.

5) Derive an affine motion predictor by setting all CPMVs to zero MV.

Note that mv ₁ is already derived in constructed affine motion predictor.

FIG. 20A shows an example of a 4-parameter affine model. FIG. 20B shows an example of a 6-parameter affine model.

FIG. 21 shows an example of an MVP for AF_INTER for inherited affine candidates.

FIG. 22 shows example MVP for AF_INTER for constructed affine candidates.

In AF_INTER mode, when 4/6-parameter affine mode is used, 2/3 control points are required, and therefore 2/3 MVD needs to be coded for these control points, as shown in FIG. 20A-20B. In an example, it is proposed to derive the MV as follows, i.e., mvd₁ and mvd₂ are predicted from mvd₀. mv ₀ =mv ₀ +mvd ₀ mv ₁ =mv ₁ +mvd ₁ +mvd ₀ mv ₂ =mv ₂ +mvd ₂ +mvd ₀

Wherein mv _(i), mvd_(i) and mv₁ are the predicted motion vector, motion vector difference and motion vector of the top-left pixel (i=0), top-right pixel (i=1) or left-bottom pixel (i=2) respectively, as shown in FIG. 20B. Please note that the addition of two motion vectors (e.g., mvA(xA, yA) and mvB(xB, yB)) is equal to summation of two components separately, that is, newMV=mvA+mvB and the two components of newMV is set to (xA+xB) and (yA+yB), respectively.

2.3.3.3 AF_MERGE Mode

When a CU is applied in AF_MERGE mode, it gets the first block coded with affine mode from the valid neighbour reconstructed blocks. And the selection order for the candidate block is from left, above, above right, left bottom to above left as shown in FIG. 23A (denoted by A, B, C, D, E in order). For example, if the neighbour left bottom block is coded in affine mode as denoted by A0 in FIG. 23B, the Control Point (CP) motion vectors mv₀ ^(N), mv₁ ^(N) and mv₂ ^(N) of the top left corner, above right corner and left bottom corner of the neighbouring CU/PU which contains the block A are fetched. And the motion vector mv₀ ^(C), mv₁ ^(C) and mv₂ ^(C) (which is only used for the 6-parameter affine model) of the top left corner/top right/bottom left on the current CU/PU is calculated based on mv₀ ^(N), mv₁ ^(N) and mv₂ ^(N). Sub-block (e.g. 4×4 block in VTM) located at the top-left corner stores mv0, the sub-block located at the top-right corner stores mv1 if the current block is affine coded. If the current block is coded with the 6-parameter affine model, the sub-block located at the bottom-left corner stores mv2; otherwise (with the 4-parameter affine model), LB stores mv2′. Other sub-blocks stores the MVs used for MC.

After the CPMV of the current CU mv₀ ^(C), mv₁ ^(C) and mv₂ ^(C) are derived, according to the simplified affine motion model Equation (1) and (2), the MVF of the current CU is generated. In order to identify whether the current CU is coded with AF_MERGE mode, an affine flag is signalled in the bitstream when there is at least one neighbour block is coded in affine mode.

FIG. 23A shows an example of candidates for AF_MERGE in a five neighboring block scenario. FIG. 23B shows an example flow of a CPMV predictor derivation process.

In examples, an affine merge candidate list is constructed with following steps:

1) Insert inherited affine candidates

Inherited affine candidate means that the candidate is derived from the affine motion model of its valid neighbor affine coded block. The maximum two inherited affine candidates are derived from affine motion model of the neighboring blocks and inserted into the candidate list. For the left predictor, the scan order is {A0, A1}; for the above predictor, the scan order is {B0, B1, B2}.

2) Insert Constructed Affine Candidates

If the number of candidates in affine merge candidate list is less than MaxNumAffineCand (e.g., 5), constructed affine candidates are inserted into the candidate list. Constructed affine candidate means the candidate is constructed by combining the neighbor motion information of each control point.

-   -   a) The motion information for the control points is derived         firstly from the specified spatial neighbors and temporal         neighbor shown in FIG. 24. CPk (k=1, 2, 3, 4) represents the         k-th control point. A0, A1, A2, B0, B1, B2 and B3 are spatial         positions for predicting CPk (k=1, 2, 3); T is temporal position         for predicting CP4.     -   The coordinates of CP1, CP2, CP3 and CP4 is (0, 0), (W, 0),         (H, 0) and (W, H), respectively, where W and H are the width and         height of current block.

FIG. 24 shows example Candidates position for affine merge mode.

The motion information of each control point is obtained according to the following priority order:

-   -   For CP1, the checking priority is B2-→B3-→A2. B2 is used if it         is available. Otherwise, if B2 is available, B3 is used. If both         B2 and B3 are unavailable, A2 is used. If all the three         candidates are unavailable, the motion information of CP1 cannot         be obtained.     -   For CP2, the checking priority is B1-→B0.     -   For CP3, the checking priority is A1-→A0.     -   For CP4, T is used.     -   b) Secondly, the combinations of controls points are used to         construct an affine merge candidate.         -   I. Motion information of three control points are needed to             construct a 6-parameter affine candidate. The three control             points can be selected from one of the following four             combinations ({CP1, CP2, CP4}, {CP1, CP2, CP3}, {CP2, CP3,             CP4}, {CP1, CP3, CP4}). Combinations {CP1, CP2, CP3}, {CP2,             CP3, CP4}, {CP1, CP3, CP4} will be converted to a             6-parameter motion model represented by top-left, top-right             and bottom-left control points.         -   II. Motion information of two control points are needed to             construct a 4-parameter affine candidate. The two control             points can be selected from one of the two combinations             ({CP1, CP2}, {CP1, CP3}). The two combinations will be             converted to a 4-parameter motion model represented by             top-left and top-right control points.         -   III. The combinations of constructed affine candidates are             inserted into to candidate list as following order:             -   {CP1, CP2, CP3}, {CP1, CP2, CP4}, {CP1, CP3, CP4}, {CP2,                 CP3, CP4}, {CP1, CP2}, {CP1, CP3}                 -   i. For each combination, the reference indices of                     list X for each CP are checked, if they are all the                     same, then this combination has valid CPMVs for                     list X. If the combination does not have valid CPMVs                     for both list 0 and list 1, then this combination is                     marked as invalid. Otherwise, it is valid, and the                     CPMVs are put into the sub-block merge list.                     3) Padding with Zero Motion Vectors

Repeated candidates: If the number of candidates in affine merge candidate list is less than 5, zero motion vectors with zero reference indices for list 0 (if P slice), and for both lists (if B slice), affine model type set to 4-parameter are insert into the candidate list, until the list is full.

More specifically, for the sub-block merge candidate list, a 4-parameter merge candidate with MVs set to (0, 0) and prediction direction set to uni-prediction from list 0 (for P slice) and bi-prediction (for B slice).

2.3.4 Current Picture Referencing

Intra block copy (IBC, or intra picture block compensation), also named current picture referencing (CPR) was adopted in HEVC screen content coding extensions (SCC). This tool is very efficient for coding of screen content video in that repeated patterns in text and graphics rich content occur frequently within the same picture. Having a previously reconstructed block with equal or similar pattern as a predictor can effectively reduce the prediction error and therefore improve coding efficiency. An example of the intra block compensation is illustrated in FIG. 25.

Similar to the design of CRP in HEVC SCC, In VVC, The use of the IBC mode is signaled at both sequence and picture level. When the IBC mode is enabled at sequence parameter set (SPS), it can be enabled at picture level. When the IBC mode is enabled at picture level, the current reconstructed picture is treated as a reference picture. Therefore, no syntax change on block level is needed on top of the existing VVC inter mode to signal the use of the IBC mode.

Main features:

-   -   It is treated as a normal inter mode. Therefore, merge and skip         modes are also available for the IBC mode. The merge candidate         list construction is unified, containing merge candidates from         the neighboring positions that are either coded in the IBC mode         or the HEVC inter mode. Depending on the selected merge index,         the current block under merge or skip mode can merge into either         an IBC mode coded neighbor or otherwise an normal inter mode         coded one with different pictures as reference pictures.     -   Block vector prediction and coding schemes for the IBC mode         reuse the schemes used for motion vector prediction and coding         in the HEVC inter mode (AMVP and MVD coding).     -   The motion vector for the IBC mode, also referred as block         vector, is coded with integer-pel precision, but stored in         memory in 1/16-pel precision after decoding as quarter-pel         precision is required in interpolation and deblocking stages.         When used in motion vector prediction for the IBC mode, the         stored vector predictor will be right shifted by 4.     -   Search range: it is restricted to be within the current CTU.     -   CPR is disallowed when affine mode/triangular mode/GBI/weighted         prediction is enabled.

2.3.5 ATMVP

In some example, when an ATMVP merge candidate is generated, the following steps are applied in order:

-   -   a. Check neighbouring blocks A1, B1, B0, A0 as shown in FIG. 2         in order, to find the first inter-coded, but not CPR-coded         block, denoted as block X;     -   b. Initialize TMV=(0,0). If there is a MV (denoted as MVn) of         block X, referring to the collocated reference picture (as         signaled in the slice header), TMV is set equal to MVn.     -   c. Suppose the center point of the current block is (x0, y0),         then locate a corresponding position of (x0,y0) as M=(x0+MV*x,         y0+MV*y) in the collocated picture. Find the block Z covering M.         -   i. If Z is intra-coded, then ATMVP is unavailable;         -   ii. If Z is inter-coded, MVZ_0 and MVZ_1 for the two lists             of block Z are scaled to (Reflist 0 index 0) and (Reflist 1             index 1) as MVdefault0, MVdefault1, and stored.     -   d. For each 8×8 sub-block, suppose its center point is (x0S,         y0S), then locate a corresponding position of (x0S, y0S) as         MS=(x0S+MV*x, y0S+MV*y) in the collocated picture. Find the         block ZS covering MS.         -   i. If ZS is intra-coded, MVdefault0, MVdefault1 are assigned             to the sub-block;         -   ii. If ZS is inter-coded, MVZS_0 and MVZS_1 for the two             lists of block ZS are scaled to (Reflist 0 index 0) and             (Reflist 1 index 0) and are assigned to the sub-block;

2.3.5.1 MV Clipping and Masking in ATMVP:

When locating a corresponding position such as M or MS in the collocated picture, it is clipped to be inside a predefined region. The CTU size is S×S, S=128. Suppose the top-left position of the collocated CTU is (xCTU, yCTU), then the corresponding position M or MS at (xN, yN) will be clipped into the valid region xCTU<=xN<xCTU+S+4; yCTU<=yN<yCTU+S.

Besides Clipping, (xN, yN) is also masked as xN=xN&MASK, yN=yN&MASK, where MASK is an integer equal to ˜(2^(N)−1), and N=3, to set the lowest 3 bits to be 0. So xN and yN must be numbers which are times of 8. (“˜” represents the bitwise complement operator).

FIG. 26 shows an example of a valid corresponding region in the collocated picture.

2.3.6 Merge List Design in VVC

There are three different merge list construction processes supported in VVC:

-   -   1) Sub-block merge candidate list: it includes ATMVP and affine         merge candidates. One merge list construction process is shared         for both affine modes and ATMVP mode. Here, the ATMVP and affine         merge candidates may be added in order. Sub-block merge list         size is signaled in slice header, and maximum value is 5.     -   2) Uni-Prediction TPM merge list: For triangular prediction         mode, one merge list construction process for the two partitions         is shared even two partitions could select their own merge         candidate index. When constructing this merge list, the spatial         neighbouring blocks and two temporal blocks of the block are         checked. The motion information derived from spatial neighbours         and temporal blocks are called regular motion candidates herein.         These regular motion candidates are further utilized to derive         multiple TPM candidates. Please note the transform is performed         in the whole block level, even two partitions may use different         motion vectors for generating their own prediction blocks.         Uni-Prediction TPM merge list size is fixed to be 5.     -   3) Regular merge list: For remaining coding blocks, one merge         list construction process is shared. Here, the         spatial/temporal/HMVP, pairwise combined bi-prediction merge         candidates and zero motion candidates may be inserted in order.         Regular merge list size is signaled in slice header, and maximum         value is 6.

2.3.6.1 Sub-Block Merge Candidate List

It is suggested that all the sub-block related motion candidates are put in a separate merge list in addition to the regular merge list for non-sub block merge candidates.

The sub-block related motion candidates are put in a separate merge list is named as ‘sub-block merge candidate list’.

In one example, the sub-block merge candidate list includes affine merge candidates, and ATMVP candidate, and/or sub-block based STMVP candidate.

2.3.6.1.1

In an example, the ATMVP merge candidate in the normal merge list is moved to the first position of the affine merge list. Such that all the merge candidates in the new list (i.e., sub-block based merge candidate list) are based on sub-block coding tools.

2.3.6.1.2 Construction Process of the Sub-Block Merge Candidate List

In some examples, a special merge candidate list, known as sub-block merge candidate list (a.k.a affine merge candidate list) is added besides the regular merge candidate list. The sub-block merge candidate list is filled with candidates in the following order:

-   -   a. ATMVP candidate (maybe available or unavailable);     -   b. Inherited Affine candidates;     -   c. Constructed Affine candidates;     -   d. Padding as zero MV 4-parameter affine model

2.3.6.2 Regular Merge List

Different from the merge list design, in VVC, the history-based motion vector prediction (HMVP) method is employed. In addition, the combined bi-predictive merge candidates described in 0 has been replaced by pairwise bi-predictive merge candidates.

2.3.6.2.1 HMVP

In HMVP, the previously coded motion information is stored. The motion information of a previously coded block is defined as an HMVP candidate. Multiple HMVP candidates are stored in a table, named as the HMVP table, and this table is maintained during the encoding/decoding process on-the-fly. The HMVP table is emptied when starting coding/decoding a new tile. Whenever there is an inter-coded non-affine/non-ATMVP block, the associated motion information is added to the last entry of the table as a new HMVP candidate. The overall coding flow is depicted in FIG. 27.

HMVP candidates could be used in both AMVP and merge candidate list construction processes. FIG. 28 depicts a modified merge candidate list construction process (highlighted in different shade). When the merge candidate list is not full after the TMVP candidate insertion, HMVP candidates stored in the HMVP table could be utilized to fill in the merge candidate list. Considering that one block usually has a higher correlation with the nearest neighboring block in terms of motion information, the HMVP candidates in the table are inserted in a descending order of indices. The last entry in the table is firstly added to the list, while the first entry is added in the end. Similarly, redundancy removal is applied on the HMVP candidates. Once the total number of available merge candidates reaches the maximal number of merge candidates allowed to be signaled, the merge candidate list construction process is terminated.

2.3.6.2.1.1 Two-Stage Pruning Procedures

There are two stages that pruning may be applied:

-   -   1) Updating the table: when one block is inter-coded and         non-affine mode, its motion information is used to update the         table, as a new HMVP candidate. Before adding the new HMVP         candidate to the table, pruning is applied.         -   If it is identical with any one of existing HMVP candidates             in the table, the duplicated one is removed from the table             and all the followed HMVP candidates are moved forward             (i.e., with index decreased by 1) and the new HMVP candidate             is added after all existing ones.         -   Otherwise, if the table is not full and it is not identical             to any existing HMVP candidates in the table, it is directly             added to the table after existing ones.     -   2) Adding HMVP candidates to the candidate list: HMVP candidates         in the HMVP tables may be used in the merge/AVMP mode.         -   For the regular merge mode, when the merge list is not full             after checking spatial and temporal merge candidates, the             HMVP candidates in a HMVP table may be further checked based             on the descending order of HMVP candidate indices.         -   For each HMVP candidate to be checked, it is compared to all             previously added merge candidates (i.e., those have been             added to the merge list), if none of existing ones are             identical the HMVP candidate, such a HMVP candidate is added             to the merge list. Otherwise, if the HMVP candidate is             identical to one of the existing ones, it is not added to             the merge list.

2.3.6.2.2 Pairwise Average Candidates

Pairwise average candidates are generated by averaging predefined pairs of candidates in the current merge candidate list, and the predefined pairs are defined as {(0, 1), (0, 2), (1, 2), (0, 3), (1, 3), (2, 3)}, where the numbers denote the merge indices to the merge candidate list. The averaged motion vectors are calculated separately for each reference list. If both motion vectors are available in one list, these two motion vectors are averaged even when they point to different reference pictures; if only one motion vector is available, use the one directly; if no motion vector is available, keep this list invalid. The pairwise average candidates replaces the combined candidates in HEVC standard. Suppose the MVs of two merge candidates are MV0=(MV0×, MV0y) and MV1=(MV1×, MV1y), then the MV of the pairwise merge candidate denoted as MV*=(MV*x, MV*y) is derived as

-   -   MV*x=(MV0×+MV1×)/2;     -   MV*y=(MV0y+MV1y)/2;

In addition, when MV0 and MV1 refer to the current picture (i.e., CPR mode), MV*x and MV*y are further rounded to remove the part with a higher precision than full pixel to make sure the integer MV is obtained:

-   -   MV*x=(MV*x/16)<<4;     -   MV*y=(MV*y/16)<<4;

It is noted that for each pair, if one of the two is coded with CPR and the other is not, such pair is disallowed to generate the pairwise average candidate.

2.3.6.2.3 Regular Merge List Construction Process

When a block is predicted using the regular merge mode, an index pointing to an entry in the regular merge candidates list is parsed from the bitstream and used to retrieve the motion information. The construction of this list is specified in the current VVC standard and can be summarized according to the following sequence of steps with changes compared to HEVC are bold faced in bigger font size:

-   -   Step 1: Initial candidates derivation         -   Step 1.1: Spatial candidates derivation         -   Step 1.2: Redundancy check/removal for spatial candidates         -   Step 1.3: Temporal candidates derivation         -   Step 1.4: HMVP candidates with redundancy check/removal             (newly introduced by VVC compared to HEVC)     -   Step 2: Virtual candidates insertion         -   Step 2.1: Creation of pairwise bi-predictive candidates             (replace the original combined bi-predictive candidates in             HEVC)         -   Step 2.2: Insertion of default motion candidates (zero             motion candidates)

2.3.7 MHIntra

With inter-intra prediction mode, multi-hypothesis prediction combines one intra prediction and one merge indexed prediction. Such a block is treated as a special inter-coded block. In a merge CU, one flag is signaled for merge mode to select an intra mode from an intra candidate list when the flag is true. For luma component, the intra candidate list is derived from 4 intra prediction modes including DC, planar, horizontal, and vertical modes, and the size of the intra candidate list can be 3 or 4 depending on the block shape. When the CU width is larger than the double of CU height, horizontal mode is exclusive of the intra mode list and when the CU height is larger than the double of CU width, vertical mode is removed from the intra mode list. One intra prediction mode selected by the intra mode index and one merge indexed prediction selected by the merge index are combined using weighted average. For chroma component, DM is always applied without extra signaling.

The weights for combining predictions are described as follow. When DC or planar mode is selected or the CB width or height is smaller than 4, equal weights are applied. For those CBs with CB width and height larger than or equal to 4, when horizontal/vertical mode is selected, one CB is first vertically/horizontally split into four equal-area regions. Each weight set, denoted as (w_intrai, w_interi), where i is from 1 to 4 and (w_intra1, w_inter1)=(6, 2), (w_intra2, w_inter2)=(5, 3), (w_intra3, w_inter3)=(3, 5), and (w_intra4, w_inter4)=(2, 6), will be applied to a corresponding region. (w_intra1, w_inter1) is for the region closest to the reference samples and (w_intra4, w_inter4) is for the region farthest away from the reference samples. Then, the combined prediction can be calculated by summing up the two weighted predictions and right-shifting 3 bits. Moreover, the intra prediction mode for the intra hypothesis of predictors can be saved for reference of the following neighboring CUs.

2.3.7.1 Signaling of Intra Prediction Modes in MHIntra-Coded Blocks

When inter-intra mode is used, one of the four allowed intra-prediction modes, DC, Planar, Horizontal and Vertical is selected and signaled. Three Most Probable Modes (MPMs) are constructed from the left and above neighbouring blocks. The intra-prediction mode of an intra-coded neighbouring block or an IIP-coded neighbouring block is treated as one MPM. If the intra-prediction mode is not one of the four allowed intra-prediction modes, it will be rounded to vertical mode or horizontal mode depending on the angular difference. The neighbouring block must be in the same CTU line as the current block.

Suppose the width and height of the current block is W and H. If W>2*H or H>2*W, then only one of the three MPMs can be used in the inter-intra mode. Otherwise, all the four valid intra-prediction modes can be used in the inter-intra mode.

It should be noted that the intra-prediction mode in inter-intra mode cannot be used to predict intra-prediction mode in a normal intra-coded block.

Inter-intra prediction can only be used when W*H>=64.

2.3.8 MMVD

In an example, ultimate motion vector expression (UMVE, also known as merge with motion vector differences (MMVD)) is presented. UMVE is used for either skip or merge modes with a proposed motion vector expression method.

UMVE re-uses merge candidate as same as those included in the regular merge candidate list in VVC. Among the merge candidates, one or multipole of them (named base candidates) can be selected, and is further expanded by the proposed motion vector expression method.

UMVE provides a new motion vector difference (MVD) representation method, in which a starting point, a motion magnitude and a motion direction are used to represent a MVD.

FIG. 29 shows an example of a UMVE search process.

FIG. 30 shows an example of a UMVE search point.

This proposed technique uses a merge candidate list as it is. But only candidates which are default merge type (MRG_TYPE_DEFAULT_N) are considered for UMVE's expansion.

Base candidate index defines the starting point. Base candidate index indicates the best candidate among candidates in the list as follows.

TABLE 1 Base candidate IDX Base candidate IDX 0 1 2 3 N^(th) MVP 1^(st) MVP 2^(nd) MVP 3^(rd) MVP 4^(th) MVP

If the number of base candidate is equal to 1, Base candidate IDX is not signaled.

Distance index is motion magnitude information. Distance index indicates the pre-defined distance from the starting point information. Pre-defined distance is as follows:

TABLE 2 Distance IDX Distance IDX 0 1 2 3 4 5 6 7 Pixel 1/4-pel 1/2-pel 1-pel 2-pel 4-pel 8-pel 16-pel 32-pel distance

Direction index represents the direction of the MVD relative to the starting point. The direction index can represent of the four directions as shown below.

TABLE 3 Direction IDX Direction IDX 00 01 10 11 x-axis + − N/A N/A y-axis N/A N/A + −

UMVE flag is signaled right after sending a skip flag or merge flag. If skip or merge flag is true, UMVE flag is parsed. If UMVE flage is equal to 1, UMVE syntax elements are parsed. But, if not 1, AFFINE flag is parsed. If AFFINE flag is equal to 1, that is AFFINE mode, But, if not 1, skip/merge index is parsed for VTM's skip/merge mode.

Additional line buffer due to UMVE candidates is not needed. Because a skip/merge candidate of software is directly used as a base candidate. Using input UMVE index, the supplement of MV is decided right before motion compensation. There is no need to hold long line buffer for this.

In current common test condition, either the first or the second merge candidate in the merge candidate list could be selected as the base candidate.

3. Examples of Problems Solved by Embodiments Disclosed in the Present Document

In the current design of VVC, three different merge list are utilized with different procedures which increases the hardware implementation cost.

-   -   1) Full pruning is applied to TPM candidates wherein each TPM         candidate to be inserted is compared to all existing ones in the         list. Such a design result in low throughput for motion vector         derivation.     -   2) Two-stage pruning operations (first stage is to update the         HMVP table that each new HMVP candidate is compared to all         existing ones in the table, the second stage is to insert a HMVP         candidate to the AMVP/merge candidate list wherein a HMVP         candidate is compared to other non-HMVP candidates (e.g.,         spatial and temporal merge candidates). Compared to the         one-stage pruning operations, such as only the second stage is         applied, the current design could bring better coding gain with         the same number of pruning operations since all the HMVP         candidates in the table are unique ones.     -   3) When adding one HMVP candidate, full pruning is applied to         compare a HMVP candidate with all existing non-HMVP candidate in         the merge list.     -   4) The TPM merge list/number of MMVD base candidates are fixed         to be 5 or 2. Such a design is not friendly for encoder         implementation with different capability.

4. Example Embodiments and Techniques

The detailed inventions below should be considered as examples to explain general concepts. These inventions should not be interpreted in a narrow way. Furthermore, these inventions can be combined in any manner.

In the following descriptions, we use ‘triangular partition mode’ to represent one as an example of the non-square/non-rectangular partition mode (TPM), and the motion vector prediction candidate inserted to TPM candidate list is named ‘TPM candidates’. It should be noted other kinds of partitions may be also applicable. The proposed methods for the TPM candidate list could be applied to any other motion candidate list of non-square/non-rectangular partitions, e.g., geometry partitions.

The proposed methods may be applied to any kinds of motion candidate list construction process (including, but not limited to AMVP/Merge list construction for regular translational motion or TPM candidates or affine motion candidates).

Suppose the (N+1) motion candidates before pruning are denoted by MCand₀, MCand₁, . . . , MCand_(N) in order.

-   -   1. The motion candidates may be classified to different         categories with different pruning methods, in one example, the         classification may be based on the associated index before         pruning and/or based on the number of available motion         candidates in the list before adding a new one.         -   a. In one example, for candidates within a category, how to             apply pruning may be the same.         -   b. In one example, the pruning methods that one category may             choose from, may include but not limited to, applying full             pruning; applying partial pruning, no pruning before adding             a candidate in this category to the candidate list.         -   c. The candidates may be classified into different             categories based on the indices of candidates. The pruning             method may be selected based on the categories.             -   i. For example, index idx within a range [StartIdx, K₀],                 full pruning may be applied when adding MCand_(idx); for                 index idx within a range [K₀+1, K₁], partial pruning may                 be applied when adding MCand_(idx); for the remaining                 index idx (i.e., within a range [K₁+1, N]), no pruning                 is applied, that is, MCand_(idx) may be directly added                 if the list is not full.             -   ii. For example, index idx within a range [StartIdx,                 K₀], either full or partial pruning may be applied when                 adding MCand_(idx); for index idx within a range [K₀+1,                 N], no pruning is applied, that is, MCand_(idx) may be                 directly added if the list is not full.             -   iii. In one example, furthermore, for those candidates                 with index within the range [StartIdx, K₀], the                 determination of applying full or partial pruning may                 further depend on the number of available candidates in                 the list.         -   d. The pruning method may be selected based on the status of             the candidate list. The status includes information such as             how many candidates are in the list and/or which kinds of             candidates are already in the list.             -   i. In one example, if there are already equal to or more                 than L available candidates in the list before adding a                 new one, partial pruning may be applied.             -   ii. In one example, if there are less than L available                 candidates in the list before adding a new one, full                 pruning may be applied.         -   e. How to apply a pruning method (including no pruning) may             depend on the number of available candidates in the list.             -   i. In one example, if there are less than L₀ available                 candidates in the list before adding a new one, full                 pruning may be applied.             -   ii. In one example, if there are already equal to or                 more than L₀ available candidates in the list before                 adding a new one, partial pruning may be applied.             -   iii. In one example, if there are already equal to or                 more than L₁ available candidates in the list before                 adding a new one, no pruning may be applied.         -   f. In one example, for the full pruning process applied to             one new candidate, such a candidate need to be compared to             all existing (previously added) candidates in the candidate             list.             -   i. If there is one existing candidate is identical or                 similar to the new candidate, this new candidate is not                 added to the list.             -   ii. If all existing candidates are not identical or                 similar to the new candidate, this new candidate may be                 added to the list, such as when the list is not full.         -   g. In one example, for the partial pruning process, a             candidate MCand_(idx) may be compared to selective existing             candidates (instead of all existing candidates) in the             candidate list.             -   i. If there is one existing candidate among the                 selective existing ones is identical or similar to the                 new candidate, this new candidate is not added to the                 list.             -   ii. If all selective existing candidates are not                 identical or similar to the new candidate, this new                 candidate may be added to the list, such as when the                 list is not full.         -   h. In one example, for the partial pruning process, a             candidate MCand_(idx) may be pruned to previously added M             candidates with consecutive indices wherein M is smaller             than idx, such as, MCand_(idx) may be compared to             (MCand_(idx-1), MCand_(idx-2), MCand_(idx-3), . . . .             MCand_(idx-M)); or MCand_(idx) may be compared to (MCand₀,             MCand₁, MCand₂, . . . , MCand_(M)). M may be fixed for all             TPM candidates that partial pruning may be required.             Alternatively, M may be changed for different candidate             index (e.g., further based on how many candidates have been             included in the list after pruning).         -   i. Alternatively, for the partial pruning process, a             candidate may be pruned to previously added candidates with             non-consecutive indices, such as MCand_(idx) may be compared             to MCand_(idx-1), MCand_(idx-3), . . . MCand_(idx-5) . . . ;         -   j. The variables (including StartIdx and/or K₀ and/or K₁             and/or L and/or L₁ and/or L₁ and/or M) may be pre-defined or             signalled in SPS/VPS/PPS/picture header/slice header/tile             group header/CTUs.         -   k. The variables (including StartIdx and/or K₀ and/or K₁             and/or L and/or L₁ and/or L₁ and/or M) may further depend on             the motion candidate list size, or slice type/picture             type/tile type/low delay check flag/block dimension.         -   l. In one example, K₀ may be set to 1. In another example K₁             may be set to K₀. In yet another example K₁ may be set to N.         -   m. In one example, StartIdx is set to 1.         -   n. The variables (including K₀ and/or K₁ and/or L and/or L₁             and/or L₁ and/or M) may be adaptively changed from block to             block.         -   o. The variable M may be further changed from one candidate             to another one. That is, the number of candidates to be             compared to (i.e., number of pruning operations) may be             different for different candidates in a candidate list.     -   2. The motion candidates mentioned above may be TPM candidates         derived from regular motion candidates (e.g., spatial/temporal         motion candidates, and/or HMVP candidates, and/or pairwise         average motion candidates).         -   a. Alternatively, the motion candidates mentioned above may             only refer to those TPM candidates derived from             uni-prediction regular motion candidates (e.g.,             spatial/temporal motion candidates, and/or HMVP candidates,             and/or pairwise average motion candidates with             uni-prediction).         -   b. Alternatively, the motion candidates mentioned above may             only refer to those TPM candidates derived from             bi-prediction regular motion candidates (e.g.,             spatial/temporal motion candidates, and/or HMVP candidates,             and/or pairwise average motion candidates with             bi-prediction).     -   3. The motion candidates mentioned above may refer to         spatial/temporal motion candidates and selective HMVP candidates         derived from one or multiple look-up tables.         -   a. In one example, all the motion information of             spatial/temporal, HMVP candidates are firstly obtained             without pruning. Then the above methods may be applied.     -   4. The above methods may be only applicable to the HMVP         candidates, i.e., how to apply pruning to HMVP candidates may         depend on the category.         -   a. In this case, the StartIdx may be defined as the number             of available merge candidates before adding any HMVP             candidates minus 1. For example, if there are M candidates             before checking any HMVP candidates (e.g., MCand₀, MCand₁, .             . . , MCand_(M-1) are spatial/temporal merge candidates).             -   i. Alternatively, furthermore, the full pruning process                 is defined as comparing a HMVP candidate with all of the                 first M candidates.             -   ii. Alternatively, furthermore, the partial pruning                 process is defined as comparing a HMVP candidate with                 partial of the first M candidates (i.e., a subset of the                 first M candidates).             -   iii. Alternatively, furthermore, different subsets may                 be used for HMVP candidates with different indices.         -   b. Alternatively, furthermore, other kinds of motion             candidates (e.g., spatial/temporal merge candidate) may             apply a different way for pruning process.     -   5. In the regular merge candidate list construction process, how         to apply the pruning process for HMVP candidates (e.g., which of         previously added merge candidates should be compared with a HMVP         candidate) may depend on where merge candidates are derived         from.         -   a. In one example, one HMVP candidate may be compared to             another merge candidate derived a given relative neighboring             block.         -   b. In one example, the selection of neighboring block which             a HMVP candidate should be compared to may be the same for             all HMVP candidates. For example, all HMVP candidates may be             compared to the spatial merge candidate derived from the             left block and/or temporal neighboring block.         -   c. HMVP candidates with different indices may be compared to             candidates derived from different blocks.     -   6. In the TPM candidate list construction process, how to apply         the pruning process (e.g., which of previously added TPM         candidates should be compared with a new candidate) may depend         on which regular motion candidates those TPM candidates are         derived from.         -   a. Alternatively, furthermore, when pruning is enabled for             TPM candidate insertion process, pruning is not applied to             regular merge candidates derivation process.     -   7. For a candidate list, it may include above-mentioned motion         candidates, after pruning if needed. In addition, it may also         add other default motion candidates.         -   a. In one example, the default motion candidates, may be             added without any pruning operations.     -   8. When a block is coded with certain types of motion         candidates, the pruning operation for the HMVP table updating         process is disabled.         -   a. In one example, a certain type is defined as the virtual             merge candidate type, which may include pairwise average             merge candidate/zero motion candidate/default motion             candidate/combined bi-predictive motion candidate.         -   b. In one example, a certain type is defined as the temporal             merge candidate type.         -   c. In one example, a certain type is defined as those motion             candidates associated with intra-inter coded blocks.         -   d. In one example, when the pruning operation for the HMVP             table updating process is disabled, the motion information             of one block and/or those derived from the motion             information of one block may be directly added to the table             after all existing HMVP candidates.         -   e. Alternatively, when the pruning operation for the HMVP             table updating process is disabled, the motion information             of one block and/or those derived from the motion             information of one block are disallowed to update the HMVP             tables.     -   9. When a block is coded with certain dimension, the pruning         operation for the HMVP table updating process is disabled.         -   a. In one example, the dimension is set to 4×4.         -   b. In one example, the dimension is set to any block size             with number of samples equal to and/or smaller than a             threshold (e.g., 64).         -   c. W>=T0 and/or H>=T1, e.g., T0 and T1 are both set to 64.     -   10. Indications of the maximum number of candidates allowed in         the TPM merge candidate list may be signaled in the bit-stream.         For example, it may be signaled in SPS/VPS/PPS/picture         header/slice header/tile group header/CTUs.         -   a. The maximum number may be directly signalled.             Alternatively, the difference between a given value K and             the maximum number may be signalled, e.g., K is set to 5 or             6.         -   b. In one example, the signaled maximum number of candidates             allowed in the TPM merge candidate (denoted as M2) cannot be             larger than the maximum number of candidates allowed in the             regular merge candidate list (denoted as M1).             -   i. For example, M1-M2 is signaled as a non-negative                 integer.         -   c. Alternatively, it is not signalled, but inferred to be             the same as M1.         -   d. In one example, truncated unary code or unary code is             applied to code such indication of maximum number of             candidates allowed in the TPM merge candidate list.     -   11. Indications of the maximum number of base merge candidates         allowed in the MMVD merge candidate list may be signaled in the         bit-stream. For example, it may be signaled in         SPS/VPS/PPS/picture header/slice header/tile group header/CTUs.         -   a. In one example, the difference between a given value K             and the maximum number may be signalled, e.g., K is set to 5             or 6.         -   b. In one example, the signaled maximum number of candidates             allowed in the MMVD merge candidate is denoted as M2, the             maximum number of candidates allowed in the regular merge             candidate list is denoted as M1.             -   i. For example, M1-M2 is signaled as a non-negative                 integer.         -   c. In one example, truncated unary code or unary code is             applied to code such indication of maximum number of base             merge candidates.     -   12. Indications of the maximum number of base merge candidates         allowed in the sub-block based MMVD merge candidate list may be         signaled in the bit-stream. For example, it may be signaled in         SPS/VPS/PPS/picture header/slice header/tile group header/CTUs.         -   a. In one example, the difference between a given value K             and the maximum number may be signalled, e.g., K is set to 5             or 6.         -   b. In one example, the signaled maximum number of candidates             allowed in the sub-block based MMVD merge candidate is             denoted as M2, the maximum number of candidates allowed in             the sub-block merge candidate list is denoted as M1.             -   i. For example, M1-M2 is signaled as a non-negative                 integer.         -   c. In one example, truncated unary code or unary code is             applied to code such indication of maximum number of base             merge candidates.     -   13. When affine mode may be allowed for TPM coded blocks, the         TPM candidates may be derived from those affine candidates         (named regular affine candidate) derived for current sub-block         merge candidate list.         -   a. In one example, if one bi-prediction affine candidate is             used, the list 0 and list 1 motion information of one affine             candidate may be used independently as two uni-prediction             affine candidates, i.e., to generate two TPM affine             candidates.     -   14. When affine mode may be allowed for TPM coded blocks, the         maximum number of candidates allowed in the TPM-affine merge         candidate list may be signaled in the bit-stream. For example,         it may be signaled in SPS/VPS/PPS/picture header/slice         header/tile group header/CTUs.         -   a. The maximum number may be directly signalled.             Alternatively, the difference between a given value K and             the maximum number may be signalled, e.g., K is set to 5 or             6.         -   b. In one example, the signaled maximum number of candidates             allowed in the TPM-affine merge candidate (denoted as M2)             cannot be larger than the maximum number of candidates             allowed in the sub-block merge candidate list or cannot be             larger than that in the affine merge candidate list (denoted             as M1).             -   i. For example, M1-M2 is signaled as a non-negative                 integer.                 -   a. In one example, it is binarized with the unary                     code.         -   c. In one example, differences between M2 and M1 may be             signalled instead.         -   d. Alternatively, it is not signalled, but inferred to be             the same as M1.

5. Embodiment 5.1 Embodiment #1: Uni-Prediction Candidate List for TPM (No Pruning for Regular Motion Candidates and Limited Pruning for TPM Candidates)

The following steps are involved to derive the TPM list and redundancy check (pruning process) for regular motion candidates are removed and TPM candidates pruning is applied.

An example is described as follows:

-   1. Obtain regular motion candidates from A₁, B₁, B₀, A₀, B₂     (corresponding to block 1-5 in FIG. 14) without pruning -   2. Obtain regular motion candidates from Col1 and/or Col2     (corresponding to block 6-7 in FIG. 14) without pruning -   3. Obtain regular motion candidates from HMVP candidates without     pruning. -   4. Set variable numCurrMrgCand=0, numCheckedCand=0; -   5. For each available regular motion candidate and numCurrMrgCand is     less than M, the following steps are performed to add TPM candidates     to the list:     -   If the regular motion candidate is uni-prediction (either from         List 0 or List 1),         -   1) If numCheckedCand is smaller than Thres,             -   pruning function PF(numCurrMrgCand) is applied to the                 regular motion candidate. If the pruning process returns                 false (i.e., no identical or similar TPM candidates                 found), such a regular motion candidate added to the                 merge list as an TPM candidate with numCurrMrgCand                 increased by 1.         -   Otherwise (numCheckedCand is equal to or larger than Thres),             it is directly added to the merge list as an TPM candidate             with numCurrMrgCand increased by 1.     -   2) numCheckedCand increased by 1.     -   If the regular motion candidate is bi-prediction, add the         following two candidate in order.     -   1) The motion information from List 0 (that is, modified to be         uni-prediction from List 0) is set to be a first new TPM         candidate.     -   2) If numCheckedCand is smaller than Thres,         -   pruning function PF(numCurrMrgCand) is invoked to the first             new TPM candidate. If the pruning process returns false,             such a first new TPM candidate added to the merge list as an             TPM candidate with numCurrMrgCand increased by 1.         -   Otherwise(numCheckedCand is equal to or larger than Thres),             it is directly added to the merge list as an TPM candidate             with numCurrMrgCand increased by 1.     -   3) numCheckedCand increased by 1.     -   4) The motion information from List 1 (that is, modified to be         uni-prediction from List 1) is set to be a second new TPM         candidate.     -   5) If numCheckedCand is smaller than Thres,         -   pruning function PF(numCurrMrgCand) is invoked to the second             new TPM candidate. If the pruning process returns false,             such a second new TPM candidate added to the merge list as             an TPM candidate with numCurrMrgCand increased by 1.         -   Otherwise (numCheckedCand is equal to or larger than Thres),             it is directly added to the merge list as an TPM candidate             with numCurrMrgCand increased by 1.     -   6) numCheckedCand increased by 1. -   6. If numCurrMrgCand is less than M, default motion candidates are     added in order till numCurrMrgCand equal to M.     -   In one example, default motion candidates are added in the         following steps in order till numCurrMrgCand equal to M:         -   Set a variable numRef=minimum (number of reference pictures             in list 0, number of reference pictures in list 1).         -   For each i being 0 . . . numRef−1,         -   i) add a default motion candidate with MV set to (0,0) and             reference picture index set to i, prediction direction set             to list 0, and numCurrMrgCand increased by 1.         -   ii) add a default motion candidate with MV set to (0,0) and             reference picture index set to i, prediction direction set             to list 1, and numCurrMrgCand increased by 1.         -   Set variable numPrevMrgCand=numCurrMrgCand.         -   For i being 0 . . . (M−numPrevMrgCand−1), add a default             motion candidate with MV set to (0,0) and reference picture             index set to 0, prediction direction set to list 0, and             numCurrMergeCand increased by 1.

Definition of the function PF(x):

-   -   Set starting index=(x>=Thres2) ? L:0;     -   Set ending index=x−1     -   For each i being [starting index, ending index], inclusively, if         the new candidate is identical (or similar) to the i-th         candidate in the list, true;     -   If none of the candidates with index within [starting index,         ending index] in the list, return false.

Note the variables Thres, Thres2, M may be pre-defined or signalled. In one example, Thres is set to 5, 6 or 7, and Thres2 is set to 4.

FIG. 31 is a block diagram of a video processing apparatus 2600. The apparatus 2600 may be used to implement one or more of the methods described herein. The apparatus 2600 may be embodied in a smartphone, tablet, computer, Internet of Things (IoT) receiver, and so on. The apparatus 2600 may include one or more processors 2602, one or more memories 2604 and video processing hardware 2606. The processor(s) 2602 may be configured to implement one or more methods described in the present document. The memory (memories) 2604 may be used for storing data and code used for implementing the methods and techniques described herein. The video processing hardware 2606 may be used to implement, in hardware circuitry, some techniques described in the present document.

FIG. 32 is a flowchart for an example method 3200 of video processing. The method 3200 includes generating (3202), during a conversion between a video comprising a current video block and a bitstream representation of the current video block, a list of motion candidates, categorizing (3204) the list of motion candidates into a number of categories of motion candidates, wherein each category is assigned a corresponding rule of pruning and performing (3206) the conversion by performing the pruning using a pruning method according to the rule of pruning to decide whether a motion candidate could be added to a final list of motion candidates and decoding the block based on the final list

It will be appreciated that several techniques have been disclosed that will benefit video encoder and decoder embodiments incorporated within video processing devices such as smartphones, laptops, desktops, and similar devices by allowing the use of ATMVP coding tool in encoding or decoding of video or images. Various embodiments and techniques may be described using the following clause-based description.

1. A method of video processing, comprising:

generating, during a conversion between a video comprising a current video block and a bitstream representation of the current video block, a list of motion candidates,

categorizing the list of motion candidates into a number of categories of motion candidates, wherein each category is assigned a corresponding rule of pruning; and

performing the conversion by performing the pruning using a pruning method according to the rule of pruning to decide whether a motion candidate could be added to a final list of motion candidates and decoding the block based on the final list.

2. The method of clause 1, wherein the categorizing is based on indexes of the motion candidates.

3. The method of any of clauses 1-2, wherein the pruning method may include full pruning, partial pruning or no-pruning prior to adding a motion candidate to the final list of motion candidates.

4. The method of any of clauses 1-3, wherein a first category includes motion candidates with indexes in a range [StartIdx, K0], a second category includes motion candidates with indexes in a range [K0+1, K1], and wherein a third category includes remaining motion candidates in a range [K1+1, N], where StartIdx, K0, K1 and N are integers.

5. The method of clause 4, wherein the rule of pruning specifies using full pruning for the first category, partial pruning for the second category and no pruning for the third category.

6. The method of clause 4, wherein the rule of pruning specifies using full pruning and partial pruning for the first category, and no pruning is applied for the second category.

7. The method of any of clauses 1-6, wherein the rule of pruning is dependent on a status of the list of motion candidates.

8. The method of clause 7, wherein the status of the list of motion candidates includes a number of motion candidates in the final list of motion candidates or a type of candidates in the list of motion candidates.

9. The method of any of clauses 1-8, wherein the motion candidates in the list of motion candidates includes at least some geometry partition mode motion candidates that are derived from regular motion candidates.

10. The method of clause 3, wherein full pruning includes, for a new candidate, comparing the new candidate to all existing candidates in the final list of motion candidates, wherein (1) the new candidate is not added to the final list of motion candidates in case that an existing candidate is identical to the new candidate, or (2) in case that the new candidate is identical to another existing candidate, adding the new candidate to the final list of motion candidates only in case that the list of motion candidates is not full.

11. The method of clause 3, wherein partial pruning includes at least one of (1) pruning a new candidate MCandidx to previously added M candidates with consecutive indices wherein M is smaller than idx, or (2) pruning a new candidates MCandidx to previously added M candidates with non-consecutive indices, where M is an integer.

12. The method of clause 11, wherein MCandidx is compared to (MCandidx-1, MCandidx-2, MCandidx-3, . . . , MCandidx-M); or MCandidx is compared to (MCand0, MCand1, MCand2, . . . , MCandM).

13. The method of clause 12, wherein M is a fixed number for all candidates.

14. The method of clause 12, wherein M is changed for different candidate indexes.

15. The method of any of clauses 4-14, wherein the list of motion candidates corresponds to history based motion vector predictor (HMVP) candidates.

16. The method of clause 15, wherein the StartIdx is one less than a number of available motion candidates prior to adding the HMVP candidates.

17. The method of any of clauses 15-16, wherein the full pruning process includes comparing the HMVP candidates with all first M candidates in the list of motion candidates.

18. A method of video processing, comprising:

generating a regular merge candidate list by applying a pruning process to history based motion vector predictor (HMVP) candidates using a rule that is based on where merge candidates are derived from, and

performing a conversion between a current video block and a bitstream representation of the current video block using the regular merge candidate list.

19. The method of clause 18, wherein the pruning process includes comparing the HMVP candidates with a merge candidate derived for a neighboring block.

20. The method of clause 19, wherein the neighboring block corresponds to a left block or a temporal neighboring block.

21. The method of any of clauses 18-20, wherein HMVP candidates having different indexes are compared with merge candidates derived from different video blocks.

22. A method of video processing, comprising:

generating a candidate list by applying a pruning process to geometry predictor mode candidates using a rule that is based on where regular motion vectors from which the geometry predictor mode candidates are derived, and

performing a conversion between a current video block and a bitstream representation of the current video block using the candidate list.

23. The method of clause 22, wherein the generating the candidate list includes refraining from applying pruning to regular merge candidates.

24. The method of any of clauses 18-23 wherein the candidate list is generated by adding default motion candidates.

25. The method of clause 24, wherein the default motion candidates are added without pruning.

26. A method of video processing, comprising:

performing a determination, for a conversion between a current video block and a bitstream representation of the current video block, to disable use of a pruning operation for history based motion vector predictor table updating processes, wherein the determination is based on a video characteristic; and

performing the conversion based on the determination to disable use of the pruning operation.

27. The method of clause 26, wherein the video characteristic is a type of motion vector or a dimension of the current video block.

28. A method of video processing, comprising:

performing a determination of a maximum number of candidates allowed for at least one of (1) a geometry prediction mode merge candidate list for a current video block, or (2) a maximum number of base merge candidates in a motion vector difference (MMVD) merge candidate list, or (3) a maximum number of merge candidates in a sub-block based MMVD merge candidate list, or (4) a maximum number of affine merge candidates in a geometry prediction mode list; and

performing, based on the determination, a conversion between the current video block and a bitstream representation of the current block,

wherein the maximum number of candidates is signaled in an indicator in the bitstream representation.

29. The method of clause 28, wherein the indicator corresponds to a difference between the maximum number of candidates allowed minus K, where K is an integer.

30. The method of clause 29, wherein K=5 or 6.

31. The method of any of clauses 28-30, wherein the maximum number of candidates allowed is M2, and M2 is not greater than M1, where M1 is a maximum number of candidates in a regular merge candidate list.

32. The method of clause 31, wherein the indicator signals a difference between M1 and M2.

33. The method of any of clauses 28-32, wherein the indicator is includes in the bitstream representation at a sequence parameter set level or a video parameter set level or a picture parameter set level or a picture header level or a slice header level or a tile group header level or a coding tree unit level.

34. A method of video processing, comprising:

performing a determination of a maximum number of candidates allowed for at least one of (1) a geometry prediction mode merge candidate list for a current video block, or (2) a maximum number of base merge candidates in a motion vector difference (MMVD) merge candidate list, or (3) a maximum number of merge candidates in a sub-block based MMVD merge candidate list or (4) a maximum number of affine merge candidates in a geometry prediction mode list; and

performing, based on the determination, a conversion between the current video block and a bitstream representation of the current block,

wherein the maximum number of candidates allowed is determined to be equal to a maximum number of candidates in a regular merge candidate list.

35. The method of any of clauses 1-34, wherein the conversion includes generating the bitstream representation from pixel values of the current video block.

36. The method of any of clauses 1-34, wherein the conversion includes generating pixel values of the current video block from the bitstream representation.

37. A video encoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses 1-34.

38. A video decoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses 1-34.

39. A computer program product stored on a non-transitory computer readable media, the computer program product including program code for carrying out the method in any one of clauses 1 to 34.

FIG. 33 is a flowchart for an example method 3300 of video processing. The method 3300 includes classifying(3302) motion candidates in a candidate list into a plurality of categories of motion candidates, wherein each category is assigned a corresponding pruning rule; performing pruning or skipping the pruning of the motion candidates in the candidate list based on the corresponding pruning rule, wherein the pruning is applied to determine whether to insert motion candidates from the candidate list into a final candidate list for a current block; performing(3306) a video processing on the current block based on the final candidate list.

FIG. 34 is a flowchart for an example method 3400 of video processing. The method 3400 includes enabling(3402) a pruning process for a geometry partition mode based motion candidate, which is to be inserted into a final candidate list for a current block, based on a pruning rule, wherein in the geometry partition mode, the current block is split into at least two partitions which are non-square partitions or non-rectangular partitions; and performing (3404) a video processing on the current block based on the final candidate list; wherein the pruning rule depends on regular merge motion candidates from which existing geometry partition mode based motion candidates in the final candidate list are derived

In some examples, the candidate list and the final candidate list are motion candidate lists for one of the modes including merge mode, advanced motion vector prediction(AMVP) mode, geometry partition (GEO) mode in which the current block is split into at least two partitions which are non-square partitions or non-rectangular partitions, triangular partition mode (TPM), affine mode.

In some examples, the motion candidates in the candidate list comprise one of spatial motion candidates derived from spatial blocks, temporal motion candidates derived from temporal blocks, history-based motion vector prediction (HMVP) candidates derived from one or multiple HMVP tables, pairwise average motion candidates, triangular motion candidates.

In some examples, the candidate list is a HMVP candidate list.

In some examples, the motion candidates in the candidate list are HMVP candidates from one or multiple HMVP tables.

In some examples, the classifying is based on indexes of the motion candidates in the candidate list.

In some examples, the pruning rule comprises at least one of a full pruning, a partial pruning and non-pruning.

In some examples, a same pruning rule is applied to motion candidates within one category.

In some examples, different pruning rules are applied to different categories.

In some examples, the plurality of categories comprises a first category including motion candidates with indexes within a range [StartIdx, K₀], StartIdx and K₀, being integers.

In some examples, either the full pruning or the partial pruning is applied to the first category.

In some examples, a determination of applying whether the full pruning or the partial pruning to the first category depends on an amount of the motion candidates in the final candidate list.

In some examples, wherein the plurality of categories further comprises a second category including motion candidates with indexes within a range [K₀+1, N], N being integers.

In some examples, non-pruning is applied to the second category.

In some examples, the non-pruning is applied to the first category, and the full pruning or the partial pruning is applied to the second category.

In some examples, the plurality of categories further comprises a second category including at least one motion candidate with at least one index within a range [K₀+1, K₁] and a third category including at least one motion candidate with at least one index within a range [K₁+1, N], K₀, K₁, and N being integers.

In some examples, the partial pruning is applied to the second category and non-pruning is applied to the third category.

In some examples, the HMVP candidates in the candidate list are classified to multiple categories, and for the first category including at least one HMVP candidate with at least one index within a range [StartIdx, K₀].

In some examples, at least one of StartIdx, K₀, and K₁ is pre-defined or signaled in at least one of Sequence parameter set(SPS), video parameter set(VPS), picture parameter set(PPS), picture header, slice header, title group header, coding tree unit(CTU).

In some examples, at least one of StartIdx, K₀, and K₁ depends on at least one of a size of the final candidate list, a slice type, a picture type, a tile type, a low delay check flag and a dimension of the current block.

In some examples, K₀=1.

In some examples, K₁=K₀ or K₁=N.

In some examples, StartIdx=1.

In some examples, the pruning rule is based on at least one of a status of the final candidate list before adding a current motion candidate, and the status of the final candidate list comprises at least one of the amount of motion candidates in the final candidate list and types of motion candidates in the final candidate list.

In some examples, full pruning is applied to the current motion candidate if the amount of the motion candidates in the final candidate list is less than a first threshold; partial pruning is applied if the amount of the motion candidates in the final candidate list is not less than the first threshold and is less than a second threshold; and non-pruning is applied if the amount of the motion candidates in the final candidate list is not less than the second threshold.

In some examples, the full pruning includes: comparing a current motion candidate to all of existing motion candidates in the final candidate list.

In some examples, the current motion candidate is inserted into the final candidate list only if the current motion candidate is neither identical nor similar to all of existing motion candidates in the final candidate list and the final candidate list is not full, or an insertion of the current motion candidate into the final candidate list is skipped if the current motion candidate is identical or similar to one of existing motion candidates in the final candidate list or the final candidate list is full.

In some examples, the partial pruning includes: comparing a current motion candidate to a subset of all of existing motion candidates in the final candidate list.

In some examples, the current motion candidate is inserted into the final candidate list only if the motion candidate is neither identical nor similar to the subset of all of existing motion candidates in the final candidate list and the final candidate list is not full; or an insertion of the current motion candidate into the final candidate list is skipped if the motion candidate is either identical or similar to one of the subset of all of existing motion candidates in the final candidate list or the final candidate list is full.

In some examples, for a HMVP candidate from a HMVP table, the full pruning includes: comparing the HMVP candidate to M motion candidates in the final candidate list.

In some examples, the HMVP candidate is inserted into the final candidate list only if the HMVP candidate is neither identical nor similar to all of the M motion candidates in the final candidate list and the final candidate list is not full; or an insertion of the HMVP candidate into the final candidate list is skipped if the HMVP candidate is identical or similar to one of the M motion candidates in the final candidate list or the final candidate list is full.

In some examples, for a HMVP candidate from a HMVP table, the partial pruning includes: comparing the HMVP candidate to a subset of M motion candidates in the final candidate list.

In some examples, the HMVP candidate is inserted into the final candidate list only if the HMVP candidate is neither identical nor similar to the subset of all of the M motion candidates in the final candidate list and the final candidate list is not full; or an insertion of the HMVP candidate into the final candidate list is skipped if the HMVP candidate is either identical or similar to one of the subset of all of the M motion candidates in the final candidate list or the final candidate list is full.

In some examples, M represents an amount of existing motion candidates in the final candidate list before inserting the HMVP candidate into the final candidate list.

In some examples, different subsets are selected for the HMVP candidates with different indexes.

In some examples, the HMVP candidate belongs to a first category including motion candidates with indexes within a range [StartIdx, K₀], StartIdx and K₀, being integers.

In some examples, for a HMVP candidate in the HMVP candidate list, the HMVP candidate is directly added to the final candidate list without prunning in response to the pruning rule being the non-pruning.

In some examples, the HMVP candidate belongs to a second category including motion candidates with indexes within a range [K₀+1, N], K₀, and N being integers.

In some examples, at least one non-HMVP candidate is pruned with pruning rules different than that for the HMVP candidates.

In some examples, the subset of existing motion candidates in the final candidate list comprises W motion candidates with consecutive or non-consecutive indexes, wherein W is smaller than an index idx of the motion candidate to be inserted.

In some examples, the W motion candidates with consecutive indexes comprise MCand_(idx-1), MCand_(idx-2), MCand_(Idx-3), . . . , MCand_(idx-W); or the W motion candidates comprise MCand₀, MCand₁, MCand₂, . . . , MCand_(W).

In some examples, the W motion candidates with non-consecutive indexes comprise MCand_(idx-1), MCand_(idx-3), MCand_(idx-5), . . . , MCand_(idx-(2W-1)).

In some examples, W is fixed for motion candidates in the sub-set of existing motion candidates.

In some examples, W is adaptive to indexes associated with the motion candidates in the final candidate list.

In some examples, W is pre-defined or signaled in at least one of Sequence parameter set(SPS), video parameter set(VPS), picture parameter set(PPS), picture header, slice header, title group header, coding tree units(CTUs).

In some examples, W depends on at least one of a size of the final candidate list, a slice type, a picture type, a tile type, a low delay check flag and a dimension of the current block.

In some examples, W depends on a specific block or on motion candidate in the candidate list.

In some examples, motion information of the motion candidates in the candidate list is obtained before the pruning is applied.

In some examples, no pruning is applied to the regular merge motion candidates.

In some examples, the method further comprises inserting default motion candidates into the final candidate list without pruning.

In some examples, the video processing comprises at least one of encoding the video block into the bitstream representation of the video block and decoding the video block from the bitstream representation of the video block.

In one example aspect, an apparatus for video processing is disclosed. The apparatus includes a processor configured to implement the methods described above.

In one example aspect, a computer program product stored on a non-transitory computer readable media is disclosed. The computer program product includes program code for carrying out the methods described above.

It will be appreciated by one of skill in the art that techniques for using motion candidate lists under various video coding scenarios are disclosed. Video blocks may be encoded into bitstream representations that include non-contiguous bits that are placed in various headers or in network adaption layer, and so on.

The disclosed and other solutions, examples, embodiments, modules and the functional operations described in this document can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this document and their structural equivalents, or in combinations of one or more of them. The disclosed and other embodiments can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more them. The term “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. A propagated signal is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this document can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random-access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

While this patent document contains many specifics, these should not be construed as limitations on the scope of any subject matter or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular techniques. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document. 

The invention claimed is:
 1. A method of video processing, comprising: constructing a motion candidate list for a current block, wherein the constructing includes determining, based on an index of a history motion vector prediction (HMVP) candidate in an HMVP list, whether to perform pruning on the HMVP candidate, and selectively performing pruning by comparing the HMVP candidate with one or more existing motion candidates in the motion candidate list to determine whether to insert the HMVP candidate from the HMVP list into the motion candidate list or skipping the pruning of the HMVP candidate based on the determining, wherein, in case the pruning is skipped, the HMVP candidate is directly inserted into the motion candidate list without comparing the HMVP candidate with the one or more existing motion candidates in the motion candidate list; and performing a conversion between the current block and a bitstream based on the motion candidate list, wherein, in case the index of the HMVP candidate is in a first index range, the pruning is performed, wherein, in case the index of the HMVP candidate is in a second index range different from the first index range, the pruning is skipped, wherein the pruning comprises a partial pruning where each one of the one or more existing motion candidates used to be compared with the HMVP candidate is derived from a given spatial neighboring block of the current block, and wherein, for performing the partial pruning, when (a) the motion candidate list comprises at least one extra motion candidate which is not included in the one or more existing motion candidates used to be compared with the HMVP candidates and (b) motion information of the HMVP candidate is not identical to motion information of the one or more existing motion candidates, the HMVP candidate is inserted directly into the motion candidate list without comparing the HMVP candidate with the at least one extra motion candidate.
 2. The method of claim 1, wherein the first index range is [X1, X2] and the second index range is [X3, X4], X1, X2, X3 and X4 being integers.
 3. The method of claim 1, wherein the second index range is [StartIdx, K] and the first index range is [K+1, N], StartIdx, K and N being integers.
 4. The method of claim 3, wherein at least one of StartIdx, K and N is pre-defined.
 5. The method of claim 1, wherein the given spatial neighboring block comprises a left spatial neighboring block of the current block and an above spatial neighboring block of the current block.
 6. The method of claim 1, wherein the one or more existing motion candidates are adjacent in the motion candidate list.
 7. The method of claim 1, wherein the pruning further comprises a full pruning that includes comparing the HMVP candidate to all of the existing motion candidates in the motion candidate list.
 8. The method of claim 7, wherein a determination of whether the full pruning or the partial pruning is applied to the current block depends on at least one of a number of all the existing motion candidates in the motion candidate list or types of the existing motion candidates in the motion candidate list.
 9. The method of claim 1, wherein whether performing the pruning or skipping the pruning is further based on a status of the motion candidate list before inserting the HMVP candidate, and wherein the status of the motion candidate list comprises at least one of a number of the existing motion candidates in the motion candidate list or types of the existing motion candidates in the motion candidate list.
 10. The method of claim 1, wherein the HMVP candidate is inserted into the motion candidate list further based on a number of all the existing candidates in the motion candidate list failing to reach a predefined value, or the HMVP candidate is not inserted into the motion candidate list based on the number of all the existing candidates in the motion candidate list reaching the predefined value.
 11. The method of claim 1, wherein the current block is coded with one of the modes including a merge mode, or a geometry partition mode.
 12. The method of claim 1, wherein the conversion comprises encoding the current block into the bitstream.
 13. The method of claim 1, wherein the conversion comprises decoding the current block from the bitstream.
 14. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to: construct a motion candidate list for a current block, wherein the constructing includes determining, based on an index of a history motion vector prediction (HMVP) candidate in an HMVP list, whether to perform pruning on the history motion vector prediction (HMVP) candidate, and selectively performing pruning by comparing the HMVP candidate with one or more existing motion candidates in the motion candidate list to determine whether to insert the HMVP candidate from the HMVP list into the motion candidate list or skipping the pruning of the HMVP candidate based on the determining, wherein, in case the pruning is skipped, the HMVP candidate is directly inserted into the motion candidate list without comparing the HMVP candidate with the one or more existing motion candidates in the motion candidate list; and perform a conversion between the current block and a bitstream based on the motion candidate list, wherein, in case the index of the HMVP candidate is in a first index range, the pruning is performed, wherein, in case the index of the HMVP candidate is in a second index range different from the first index range, the pruning is skipped, wherein the pruning comprises a partial pruning where each one of the one or more existing motion candidates used to be compared with the HMVP candidate is derived from a given spatial neighboring block of the current block, and, wherein, for performing the partial pruning, when (a) the motion candidate list comprises at least one extra motion candidate which is not included in the one or more existing motion candidates used to be compared with the HMVP candidates and (b) motion information of the HMVP candidate is not identical to motion information of the one or more existing motion candidates, the HMVP candidate is inserted directly into the motion candidate list without comparing the HMVP candidate with the at least one extra motion candidate.
 15. The apparatus of claim 14, wherein the first index range is [X1, X2] and the second index range is [X3, X4], X1, X2, X3 and X4 being integers, or the first index range is [StartIdx, K] and the second index range is [K+1, N], StartIdx, K and N being integers, and wherein at least one of StartIdx, K and N is pre-defined.
 16. A non-transitory computer-readable storage medium storing instructions that cause a processor to: construct a motion candidate list for a current block, wherein the constructing includes determining, based on an index of a history motion vector prediction (HMVP) candidate in an HMVP list, whether to perform pruning on the history motion vector prediction (HMVP) candidate, and selectively performing pruning by comparing the HMVP candidate with one or more existing motion candidates in the motion candidate list to determine whether to insert the HMVP candidate from the HMVP list into the motion candidate list or skipping the pruning of the HMVP candidate based on the determining, wherein, in case the pruning is skipped, the HMVP candidate is directly inserted into the motion candidate list without comparing the HMVP candidate with the one or more existing motion candidates in the motion candidate list; and perform a conversion between the current block and a bitstream based on the motion candidate list, wherein, in case the index of the HMVP candidate is in a first index range, the pruning is performed, wherein, in case the index of the HMVP candidate is in a second index range different from the first index range, the pruning is skipped, wherein the pruning comprises a partial pruning where each one of the one or more existing motion candidates used to be compared with the HMVP candidate is derived from a given spatial neighboring block of the current block, and wherein, for performing the partial pruning, when (a) the motion candidate list comprises at least one extra motion candidate which is not included in the one or more existing motion candidates used to be compared with the HMVP candidates and (b) motion information of the HMVP candidate is not identical to motion information of the one or more existing motion candidates, the HMVP candidate is inserted directly into the motion candidate list without comparing the HMVP candidate with the at least one extra motion candidate.
 17. The non-transitory computer-readable storage medium of claim 16, wherein the first index range is [X1, X2] and the second index range is [X3, X4], X1, X2, X3 and X4 being integers, or the first index range is [StartIdx, K] and the second index range is [K+1, N], StartIdx, K and N being integers, wherein at least one of StartIdx, K and N is pre-defined.
 18. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises: constructing a motion candidate list for a current block, wherein the constructing includes determining, based on an index of a history motion vector prediction (HMVP) candidate in an HMVP list, whether to perform pruning on the HMVP candidate, and selectively performing pruning by comparing the HMVP candidate with one or more existing motion candidates in the motion candidate list to determine whether to insert the HMVP candidate from the HMVP list into the motion candidate list or skipping the pruning of the HMVP candidate based on the determining, wherein, in case the pruning is skipped, the HMVP candidate is directly inserted into the motion candidate list without comparing the HMVP candidate with the one or more existing motion candidates in the motion candidate list; and generating the bitstream based on the motion candidate list, wherein, in case the index of the HMVP candidate is in a first index range, the pruning is performed, wherein, in case the index of the HMVP candidate is in a second index range different from the first index range, the pruning is skipped, wherein the pruning comprises a partial pruning where each one of the one or more existing motion candidates used to be compared with the HMVP candidate is derived from a given spatial neighboring block of the current block, and wherein, for performing the partial pruning, when (a) the motion candidate list comprises at least one extra motion candidate which is not included in the one or more existing motion candidates used to be compared with the HMVP candidates and (b) motion information of the HMVP candidate is not identical to motion information of the one or more existing motion candidates, the HMVP candidate is inserted directly into the motion candidate list without comparing the HMVP candidate with the at least one extra motion candidate.
 19. The non-transitory computer-readable recording medium of claim 18, wherein the first index range is [X1, X2] and the second index range is [X3, X4], X1, X2, X3 and X4 being integers, or the first index range is [StartIdx, K] and the second index range is [K+1, N], StartIdx, K and N being integers, wherein at least one of StartIdx, K and N is pre-defined. 